#include <stdio.h>
#include <ctype.h>

int main() {
    char str[100];
    scanf("%s", str);
    
    char *ptr;
    
    while (*p != '\0') {
        if (isdigit(*p)) {
            printf("Invalid input");
            return 0;
        }
        p++;
    }
    
    p = str;
    
    char min = *p;
    while (*p != '\0') {
        if (*p < min) {
            min = *p;
        }
        p++;
    }
    
    printf("%c", min);
    return 0;
}