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

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

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

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

int main() {
    char number[1024] = {0};
    char read = 1;
    int length = 0;

    while(((int)read) > 0) {
        scanf("%c", &read);
        if(read == '\n')
            break;
        length++;
        number[length - 1] = read;
    }

    unsigned long 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);
        free(dgsm);
    }
    printf("%d", digit_sum);
    free(number);
    return 0;
}