#include <bits/stdc++.h>
using namespace std;

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