#include <bits/stdc++.h>

using namespace std;

int main(){
  int b[9];
  int i = 0;
  for(i = 0; i < 9; i++){
    cin >> b[i];
    if(i > 0){
      if(b[i] != b[i-1]+1){
        cout << b[i] - 1 << endl;
        break;
      }
    }else if(b[i] == 2){
      cout << 1 << endl;
      break;
    }
  }
  if(i == 9){
    cout << 10 << endl;
  }
  
  return 0;
}