#include <iostream>
using namespace std;

int main(){
    int arr[9];
    for (int i=0;i<9;i++){
        cin>>arr[i];
    }
    int missID=0;
    for (int x=0;x<10;x++){
        if(arr[x]!=x+1){
            missID=x+1;
            break;
        }
    }
    cout<<missID<<endl;
}