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


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