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