#include<bits/stdc++.h>
using namespace std;
int main(){
	int n;
	cin>>n;
	int ans=0;
	while(n>0){
		ans+=n%10;
		n/=10;
	}
	cout<<ans;
	return 0;
}

while不会用吗

#include<bits/stdc++.h> using namespace std; int main() { int s,ge,shi,bai,qian; cin>>s; ge=s%10; shi=s%100/10; bai=s/100%10; qian=s/1000; cout<<ge+shi+bai+qian; return 0; }

#include<iostream>
using namespace std;
int main()
{
int x;
cin>>x;
cout<<x-x/1000-x%1000/100*10-x%1000%100/10*100-x%1000%100%10*1000;
}