#include <bits/stdc++.h>
using namespace std;
#define LL long long int
int main () {
    vector<int>a(9);
    for (int i=0; i<9; i++) cin >> a[i];
    int ans=0;
    for (int i=1; i<9; i++) {
        if (a[i]-a[i-1] != 1) {
            ans=(a[i]+a[i-1])/2;
            break;
        }
    }
    cout << ans << "\n";
}