#include <iostream>
#include<vector>
using namespace std;

int main() {
  int n;
  cin >> n;
  vector<int>rice(n);
  int left = (n+1)*n/2;
  for(int i = 0;i<n-1;i++){
    cin >> rice[i];
    left-=rice[i];
  }
  cout << left <<endl;
}

等差数列求和 Sn = (n+1)*n/2