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