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