#The Biggest Water Problem#

#include <stdio.h>
int fun(int n) {
    int sum = 0;
    while (n) {
        sum += n % 10;
        n /= 10;
    }
    if (sum > 9) {
        return fun(sum);
    }
    return sum;
}
int main() {
    int n;
    scanf("%d", &n);
    printf("%d\n", fun(n));
    return 0;
}