#include <stdio.h>

int main() {
    int a = 0, result = 0;
    scanf("%d", &a);
    while (a >= 1) {
        // printf("%d\n", a % 10);
        result += a % 10;
        a /= 10;
        if (a == 0 && result >= 10) {
            a = result;
            result = 0;
        }
    }

    printf("%d", result);
    return 0;
}