#include <stdio.h>
#include <algorithm>
using namespace std;
using ll = long long;

int main() {
	int b, p = 0, res = 0;
	for (int i = 0; i < 9; i++) {
		scanf("%d", &b);
		if (p + 1 != b)
			res = p + 1;
		p = b;
	}
	printf("%d\n", res);
}