#include<bits/stdc++.h>
using namespace std;
int main(){
    string a;
	int n=0;
	cin>>a;
	for(int i=0;i<a.size();i++) if(a[i]=='1') n++;
	cout<<n; 
}
   
/**************************************************************
	Problem: 1934
	User: panyuchen
	Language: C++
	Result: Accepted
	Time:71 ms
	Memory:2076 kb
****************************************************************/