#include <bits/stdc++.h>
void dout() { printf("\n"); }
template <typename Head, typename... Tail>
void dout(Head H, Tail... T) {}
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  int a = 0, b = 0;
  for (int i = 1; i < n + 1; ++i) {
    a += i;
  }
  for (int i = 0; i < n - 1; ++i) {
    int t;
    scanf("%d", &t);
    b += t;
  }
  printf("%d", a - b);
  return 0;
}
