#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  scanf("%d", &n);
  long long s = (long long)(n + 1) * n / 2;
  for (int i = 0; i < n - 1; ++i) {
    int t;
    scanf("%d", &t);
    s -= t;
  }
  printf("%I64d\n", s);
  return 0;
}
