text: /**
   *    author: etohirse
   *    created: 06.01.2021 14:54:02
   **/
  #include <bits/stdc++.h>
  
  typedef unsigned long long ll;
  
  int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
  
    ll n, s(0);
    std::cin >> n;
    for (int i = 0u; i < n - 1; ++i) {
      ll x;
      std::cin >> x;
      s += x;
    }
    std::cout << n * (n + 1) / 2 - s;
    return 0;
  }
  