diff --git a/src/Phpml/Classifier/Classifier.php b/src/Phpml/Classifier/Classifier.php
index ea2bbf1..1d2362e 100644
--- a/src/Phpml/Classifier/Classifier.php
+++ b/src/Phpml/Classifier/Classifier.php
@@ -1,4 +1,5 @@
 <?php
+declare(strict_types=1);
 
 namespace Phpml\Classifier;
 
@@ -9,7 +10,7 @@ interface Classifier
      * @param array $features
      * @param array $labels
      */
-    public function train($features, $labels);
+    public function train(array $features, array $labels);
 
     /**
      * @param mixed $feature
diff --git a/src/Phpml/Classifier/KNearestNeighbors.php b/src/Phpml/Classifier/KNearestNeighbors.php
new file mode 100644
index 0000000..ac4bf31
--- /dev/null
+++ b/src/Phpml/Classifier/KNearestNeighbors.php
@@ -0,0 +1,54 @@
+<?php
+declare(strict_types=1);
+
+namespace Phpml\Classifier;
+
+class KNearestNeighbors implements Classifier
+{
+    /**
+     *
+     * @var int
+     */
+    private $k;
+
+    /**
+     * @var array
+     */
+    private $features;
+
+    /**
+     * @var array
+     */
+    private $labels;
+
+    /**
+     * @param int $k
+     */
+    public function __construct(int $k = 3)
+    {
+        $this->k = $k;
+        $this->features = [];
+        $this->labels = [];
+    }
+
+
+    /**
+     * @param array $features
+     * @param array $labels
+     */
+    public function train(array $features, array $labels)
+    {
+        $this->features = $features;
+        $this->labels = $labels;
+    }
+
+    /**
+     * @param mixed $feature
+     * @return mixed
+     */
+    public function predict($feature)
+    {
+
+    }
+
+}
diff --git a/src/Phpml/Classifier/NaiveBayes.php b/src/Phpml/Classifier/NaiveBayes.php
index 7207409..8e31eff 100644
--- a/src/Phpml/Classifier/NaiveBayes.php
+++ b/src/Phpml/Classifier/NaiveBayes.php
@@ -1,10 +1,27 @@
 <?php
+declare(strict_types=1);
 
 namespace Phpml\Classifier;
 
-abstract class NaiveBayes implements Classifier
+class NaiveBayes implements Classifier
 {
 
+    /**
+     * @param array $features
+     * @param array $labels
+     */
+    public function train(array $features, array $labels)
+    {
 
+    }
+
+    /**
+     * @param mixed $feature
+     * @return mixed
+     */
+    public function predict($feature)
+    {
+
+    }
 
 }
diff --git a/src/Phpml/Classifier/NaiveBayes/GaussianNaiveBayes.php b/src/Phpml/Classifier/NaiveBayes/GaussianNaiveBayes.php
deleted file mode 100644
index 33117fa..0000000
--- a/src/Phpml/Classifier/NaiveBayes/GaussianNaiveBayes.php
+++ /dev/null
@@ -1,26 +0,0 @@
-<?php
-
-use Phpml\Classifier\NaiveBayes;
-
-class GaussianNaiveBayes extends NaiveBayes
-{
-
-    /**
-     * @param array $features
-     * @param array $labels
-     */
-    public function train($features, $labels)
-    {
-
-    }
-
-    /**
-     * @param mixed $feature
-     * @return mixed
-     */
-    public function predict($feature)
-    {
-
-    }
-
-}
diff --git a/src/Phpml/Exception/InvalidArgumentException.php b/src/Phpml/Exception/InvalidArgumentException.php
new file mode 100644
index 0000000..c88ab50
--- /dev/null
+++ b/src/Phpml/Exception/InvalidArgumentException.php
@@ -0,0 +1,17 @@
+<?php
+declare(strict_types = 1);
+
+namespace Phpml\Exception;
+
+class InvalidArgumentException extends \Exception
+{
+
+    /**
+     * @return InvalidArgumentException
+     */
+    public static function parametersSizeNotMatch()
+    {
+        return new self('Size of parameters not match');
+    }
+
+}
diff --git a/src/Phpml/Metric/Distance.php b/src/Phpml/Metric/Distance.php
new file mode 100644
index 0000000..e1e64c6
--- /dev/null
+++ b/src/Phpml/Metric/Distance.php
@@ -0,0 +1,36 @@
+<?php
+declare(strict_types = 1);
+
+namespace Phpml\Metric;
+
+use Phpml\Exception\InvalidArgumentException;
+
+class Distance
+{
+
+    /**
+     * @param array $a
+     * @param array $b
+     *
+     * @return float
+     *
+     * @throws InvalidArgumentException
+     */
+    public static function euclidean(array $a, array $b): float 
+    {
+        if(count($a) != count($b))
+        {
+            throw InvalidArgumentException::parametersSizeNotMatch();
+        }
+
+        $distance = 0;
+
+        for($i=0; $i<count($a); $i++)
+        {
+            $distance += pow($a[$i] - $b[$i], 2);
+        }
+
+        return sqrt($distance);
+    }
+
+}