Initial data commit

Task/Fibonacci-sequence/C-sharp/fibonacci-sequence-3.cs

@@ -0,0 +1,13 @@
+public static ulong Fib(uint x) {
+    if (x == 0) return 0;
+
+    ulong prev = 0;
+    ulong next = 1;
+    for (int i = 1; i < x; i++)
+    {
+        ulong sum = prev + next;
+        prev = next;
+        next = sum;
+    }
+    return next;
+}