#include <bits/stdc++.h>

#define ll long long

using namespace std;

const int MAXN = 10000;
string s;
vector<bool> zecovi;
bool hasChange = true;
vector<bool> zecovi2;
int sol = 0;

int main() {

	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	cin >> s;
	
	for (int i = 0; i < s.size(); i++) {
		if (s[i] == '1') zecovi.push_back(true);
		else zecovi.push_back(false);
	}
	
	do {
		hasChange = false;
		zecovi2 = zecovi;
		int k = MAXN, pos = -1;
		bool inOnes = false;
		
		for (int i = 0; i < zecovi.size(); i++) {
			if (zecovi[i] && !inOnes) {
				pos = i;
				inOnes = !inOnes;
			}
			
			if (!zecovi[i] && inOnes) {
				k = min(k, i - pos);
				inOnes = !inOnes;
			}
		}
		
		if (k == MAXN) {
			cout << -1;
			return 0;
		}
		
		for (int i = 0; i < zecovi.size() - k; i++) {
			//cout << i << " " << k << " " << zecovi.size() << " " << zecovi2.size() << endl;
			if (zecovi[i]) {
				zecovi2[i + k] = true;
				hasChange = true;
			}
		}
		
		sol++;
		zecovi = zecovi2;
	} while(hasChange);

	cout << sol;

	return 0;
}