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