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

/**************************************************************
	Problem: 1934
	User: fzy001
	Language: C++
	Result: Wrong Answer
****************************************************************/