#include <stdio.h>
#include <math.h>

int main() {
    int number = 0;
    scanf("%d", &number);
    
    int length = 0;
    while ((number % (int)pow(10, length)) != number) {
        length ++;
    }

    int digital = 0, maxDigital = 0;
    for (int i = 0; i < length; ++i) {
        digital = number % 10;
        maxDigital = digital > maxDigital ? digital : maxDigital;
        number /= 10;
    }

    printf("%d", maxDigital);
}