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