// editor3
#include<stdio.h>
#include<ctype.h>

int main(){
    char str[101];
    scanf("%1000s",str);
    cahr *ptr = str;
    while(*ptr != ''){
        if(!isalpha((unsigned char)*ptr)){
            printf("Invalid input");
            return 0;
        }
        ptr++;
    }
    ptr = str;
    cahr minc=*ptr;
    ptr++;
    while(*ptr != ''){
        if(*ptr <minc){
            minc = *ptr;
        }
        ptr++;
    }
    printf("%c",minc);
    return 0;
}