#include <iostream>
#include <sstream>
using namespace std;

int main() {
	int n;
	char k;

	istringstream input{"3 2\n2 3\n0"}; // {"3 1\n1 2\n1 3\n0"};
	input >> n;
	input >> k;
	
    bool isKGreatest = true;
    char first;
    char second;

    while(input >> first >> second) {
        n--;

        if(second == k){
            isKGreatest = false;
            break;
        }
    }

    if(n > 1) {
        isKGreatest = false;
    }
	
	cout << (isKGreatest ? "YES\n" : "NO\n");
}