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