#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define o(a) cout << a << endl
#define int long long
using namespace std;
typedef pair<int, int> P;

signed main(){
    int f[10] = {0};
    rep(i, 0, 9){
        int a;
        cin >> a;
        f[a - 1]++;
    }
    int ans = -1;
    rep(i, 0, 10){
        if(f[i] == 0) ans = i + 1;
    }
    cout << ans << endl;
}