#include <iostream> #include <string>

char findFirstCharacter(int k, const std::string& s) { int count = 1; char prev = s[0];

for (int i = 1; i < s.length(); i++) {
    if (s[i] == prev) {
        count++;
    } else {
        prev = s[i];
        count = 1;
    }

    if (count >= k) {
        return prev;
    }
}

return 'N'; // 如果没有符合条件的字符，返回'N'

}

int main() { int k; std::string s;

std::cin >> k;
std::cin.ignore(); // 忽略换行符
std::getline(std::cin, s);

char result = findFirstCharacter(k, s);
if (result == 'N') {
    std::cout << "No" << std::endl;
} else {
    std::cout << result << std::endl;
}

return 0;

}