The Biggest Water Proble

#include <stdio.h>

int main() {
    int num = 0;

    scanf("%d", &num);
    while (num >= 10) 
    {
        int sum = 0;
        while (num)
        {
            // 将num的每一位上的数字进行相加
            sum += (num % 10);
            num /= 10;
        }
        num = sum;
    }
    printf("%d", num);

    return 0;
}