#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, r;
  cin >> n;
  r = n * (n + 1) / 2;
  for (long long x, i = 1; i < n; i++) {
    cin >> x;
    r -= x;
  }
  cout << r;
  return 0;
}
