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

int main() {
    char str[101];  // Max 100 characters + '\0'
    scanf("%s", str);

    // Check for invalid input (digits)
    for (int i = 0; str[i] != '\0'; i++) {
        if (!isalpha(str[i])) {
            printf("Invalid input\n");
            return 0;
        }
    }

    // Use pointer to find minimum ASCII character
    char *ptr = str;
    char minChar = *ptr;

    while (*ptr != '\0') {
        if (*ptr < minChar) {
            minChar = *ptr;
        }
        ptr++;
    }

    printf("%c\n", minChar);
    return 0;
}