#include <bits/stdc++.h>

using namespace std;

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	string s, t;
	cin >> s >> t;
	int n = s.size();
	if (t.size() == 1) {
		for (char c : s) {
			if (c == t[0]) {
				cout << "-1\n";
				return 0;
			}
		}
		cout << "0\n";
		return 0;
	}
	if (t == "11") {
		t = "00";
		for (auto &c : s) {
			if (c == '0') {
				c = '1';
			} else {
				c = '0';
			}
		}
	}
	if (t == "10") {
		t = "01";
		for (auto &c : s) {
			if (c == '0') {
				c = '1';
			} else {
				c = '0';
			}
		}
	}
	if (t == "00") {
		int cnt = 0;
		int ans = 0;
		int len = 0;
		for (int i = 0; i < n; ++i) {
			if (s[i] == '1') {
				len = 0;
				++cnt;
			} else {
				++len;
				if (i == n - 1 || s[i + 1] == '1') {
					ans += len - 1;
				}
			}
		}
		if (2 * cnt + 1 < n) {
			cout << "-1\n";
		} else {
			cout << ans << "\n";
		}
		return 0;
	}
	if (t == "01") {
		int cnt = 0;
		for (int i = 0; i < n - 1; ++i) {
			if (s[i] == '0' && s[i + 1] == '1') {
				++cnt;
			}
		}
		cout << cnt << "\n";
	}
	return 0;
}