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

int main() {

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