#include <iostream>
#include <cstdint>

using namespace std;

int main()
{
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	
	static char S[2000001];
	cin >> S;

	int32_t i, l = 0;
	int64_t ans = 0;
	for (i = 0; ; ++i)
		switch (S[i])
		{
		case '0':
			ans += (i - l) >> 1;
			l += 1 + ((i - l) & 1);
			break;

		case '\0':
			cout << ans << '\n';
			return 0;
		}

	cerr << "Buffer overflow...\n";
	return -1;
}