#include <iostream>
using namespace std;
string a;
int check(string a)
{
	int cnt = 0;
	int len = a.size();
	for(int i = 1;i <= len;i++)
	{
		if(a[i] == '1')
		{
			cnt++;
		}
	}
	return cnt;
}
int main()
{
	cin >> a;
	cout << check(a);
}