using System;
public class Program
{
public void Main()
Console.WriteLine(FindMissing(new int[] {19,14,2,17,4,12,20,7,16,9,13,8,11,18,3,6,10,1,15}));
}
private int FindMissing(int[] input)
var sum = 0;
for(int i = 0; i < input.Length; i++) {
sum += input[i];
var n = input.Length + 1;
return (n * (n + 1) / 2) - sum;