#include <bits/stdc++.h>
int main() {
  long long n, i, j, c;
  scanf("%I64d", &n);
  int a[n - 1];
  for (i = 0, c = 0; i < n - 1; i++) {
    scanf("%d", &a[i]);
    c += a[i];
  }
  j = n * (n + 1) / 2;
  printf("%I64d", j - c);
  return 0;
}
