#include<bits/stdc++.h>
using namespace std;
int main()
{
	string s;
	int sum = 0;
	cin >> s;
	for( int i = 0; i < s.size(); i++)
	{
		if(s[i] == '1')
		{
			sum++;
		}
	}
	cout << sum;
	return 0;
}
/**************************************************************
	Problem: 1934
	User: yangwanning
	Language: C++
	Result: Accepted
	Time:67 ms
	Memory:2076 kb
****************************************************************/