#include<stdio.h>
#include<ctype.h>
int main()
{
    char str[100];
    char *ptr;
    char minChar;
    scanf("%s" , str);
    ptr = str;
    minChar = *ptr;
    while(*ptr !="\0"){
        if(!isalpha(*ptr)){
            printf("Invalid input");
            return 0;
        }
        ptr++;
    }
    ptr=str;
    while(*ptr!='\0'){
        if(*ptr<minChar)
        minChar=*ptr;
        ptr++;
    }
    printf("%c",minChar);
    return 0;
}