#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  scanf("%d", &N);
  int res = 0;
  for (int i = 1; i <= N; ++i) {
    res ^= i;
  }
  for (int i = 0; i < N - 1; ++i) {
    int A;
    scanf("%d", &A);
    res ^= A;
  }
  printf("%d\n", res);
  return 0;
}
