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