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