#include<iostream>
#include<algorithm>

using namespace std;

int main(){

  int k[9],j[10],n;

  for(int i = 0; i < 10;i++)
    j[i] = i+1;

  for(int i = 0; i < 9;i++)
    cin >> k[i];

    int i;
    for(i = 0; i < 10;i++)
      if(j[i]!=k[i])
        break;

  cout << j[i] << endl;

  return 0;
}