#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");
}

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