#include <iostream>
#include <algorithm>

inline bool  is_prime(int n) {
    for (int i = 2; i < n; i++) {
        if (n % i == 0)
            return false;
    }
    return true;
}

int main() {
    std::string s;
    std::cin >> s;
    int  maximum = 0;
    for (int  i = 0; i < s.length(); i++) {
        for (int c = 1; c < s.length() - i; c++) {
            int num = std::stoi(s.substr(i, c));
            if (is_prime(num))
                maximum = std::max(maximum, num);
        }
    }
    std::cout << maximum;
    return 0;
}