#include <stdio.h>
#include <malloc.h>
#include <string.h>

char* int_to_string(const int a) {
    int temp = a;
    int length = 0;
    while (temp > 10) {
        temp /= 10;
        length++;
    }

    char* string = calloc(length, sizeof(char));
    sprintf(string, "%d%c", a, 0);
    return string;
}

int calc_digit_sum(const char* string, int length) {
    int sum = 0;
    for (int i = 0; i < length; i++)
        sum += string[i] - 0x30;
    return sum;
}

int main() {
    int in_number;
    scanf("%d", &in_number);


    char* number = int_to_string(in_number);
    int length = strlen(number);
    int digit_sum = calc_digit_sum(number, length);

    while(digit_sum > 9) {
        char* dgsm = int_to_string(digit_sum);
        int dgsm_length = strlen(dgsm);

        digit_sum = calc_digit_sum(dgsm, dgsm_length);

    }

    printf("%d", digit_sum);

    return 0;
}