知识点：

函数：函数。 

#include <iostream>
using namespace std;

int fun(int n);

int main() {
    int n;
    int ret;

    cin >> n;
    ret = n;
    do {
        ret = fun(ret);
    } while (ret >= 10);

    cout << ret;

    return 0;
}

int fun(int n) {
    int sum = 0;
    int temp;

    while (n > 0) {
        temp = n % 10;
        sum += temp;
        n /= 10;
    }

    return sum;
}