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