#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, sum = 0, sum1 = 0;
  int x;
  cin >> n;
  for (int i = 0; i < (int)n - 1; ++i) {
    cin >> x;
    sum += x;
  }
  for (int i = 1; i <= n; i++) sum1 += i;
  cout << sum1 - sum << endl;
  return 0;
}
