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

int main() {
    char str[100];
    char *p;
    char minChar;


    scanf("%s", str);


    for (p = str; *p != '\0'; p++) {
        if (isdigit(*p)) {
            printf("Invalid input");
            return 0;
        }
    }
    minChar = str[0];
    for (p = str + 1; *p != '\0'; p++) {
        if (*p < minChar) {
            minChar = *p;
        }
    }
    printf("%c", minChar);
return 0;
}