Create docstring, test cases and fix a small bug

Create test cases and fix a bug where the program stop when x or y is less than or equal to 0.

lcm.py

@@ -1,4 +1,34 @@
 def lcm(x, y):
+    """
+        Find least common multiple of 2 positive integers.
+        :param x: int - first integer
+        :param y: int - second integer
+        :return: int - least common multiple
+
+        >>> lcm(8, 4)
+            8
+        >>> lcm(5, 3)
+            15
+        >>> lcm(15, 9)
+            45
+        >>> lcm(124, 23)
+            2852
+        >>> lcm(3, 6)
+            6
+        >>> lcm(13, 34)
+            442
+        >>> lcm(235, 745)
+            35015
+        >>> lcm(65, 86)
+            5590
+        >>> lcm(0, 1)
+            -1
+        >>> lcm(-12, 35)
+            -1
+    """
+    if x <= 0 or y <= 0:
+        return -1
+
     if x > y:
         greater_number = x
     else: