#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef long long ll;
const int INF = 1000000000; // 10^9
int main() {

    const int N = 9;
    vector<int> v(N);
    rep(i, N) cin >> v[i];

    for (int i = 0; i < N; i++){
        int x = v[i];
        x--;
        if(x != i) {
            cout << x << endl;
            return 0;
        }
    }
}