#include <bits/stdc++.h>
int main() {
  long long int i, n, m, p, q;
  p = 0;
  scanf("%lld", &n);
  m = (n * (n + 1)) / 2;
  for (i = 1; i < n; i++) {
    scanf("%lld", &q);
    p = p + q;
  }
  printf("%lld\n", m - p);
  return 0;
}
