#include <bits/stdc++.h>
int main() {
  int n, t;
  while (scanf("%d", &n) != EOF) {
    double sum = 0;
    for (int i = 1; i <= n; i++) sum += i;
    for (int i = 0; i < n - 1; i++) {
      scanf("%d", &t);
      sum -= t;
    }
    printf("%.0f\n", sum);
  }
}
