The Biggest Water Problem

地址

#include<bits/stdc++.h>
using namespace std;

typedef long long ll;

int main()
{
    ll n;
    cin>>n;
    ll sum=0;
    while(n>10){
        ll sum=n;
        ll d=0;
        while(sum){
            d+=sum%10;
            sum/=10;
        }
        n=d;
    }
    cout<<n;
    return 0;
}