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