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