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

int main(){
    char str[101];
    scanf("%s", str);
    
    char *ptr = str;
    char min_char = *ptr;
    
    while(*ptr != '\0') {
        if (isdigit(*ptr)){
            printf("Invalid input\n");
            return 0;
        }
        if(*ptr < min_char){
            min_char = *ptr;
        }
        ptr++
    }
    printf("%c\n", min_char);
    return 0;
    
}