#include<bits/stdc++.h>
using namespace std;
int main(){
	string s;
	cin>>s;
	int d=0;
	for(int i=0;i<s.size();i++){
		if(s[i]=='1'){
			d++;
		}
	}
	cout<<d;
	return 0;
}

/**************************************************************
	Problem: 1934
	User: zzz
	Language: C++
	Result: Accepted
	Time:67 ms
	Memory:2076 kb
****************************************************************/