#include <iostream>
using namespace std;

int main()
{
	int B[9];

	for (int i = 0; i < 9; i++)
	{
		cin >> B[i];
	}

	for (int i = 0; ; i++)
	{
		if (B[i + 1] - B[i] != 1)
		{
			cout << B[i] + 1 << endl;
			break;
		}
	}

}