diff --git a/cs/find-the-duplicate-number.cs b/cs/find-the-duplicate-number.cs
new file mode 100644
index 0000000..a16ddfa
--- /dev/null
+++ b/cs/find-the-duplicate-number.cs
@@ -0,0 +1,19 @@
+public class Solution {
+    public int FindDuplicate(int[] nums) {
+        int slow = nums[0];
+        int fast = nums[0];
+
+        do {
+            slow = nums[slow];
+            fast = nums[nums[fast]];
+        } while (fast != slow);
+
+        slow = nums[0];
+        while (slow != fast) {
+            slow = nums[slow];
+            fast = nums[fast];
+        }
+
+        return slow;
+    }
+}