#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define flt double
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()

mt19937 rnd(234);
const ll inf = 1e18;

int n, m;
vector<int> s, t;

ll solve() {
    if (t[0] == 1) {
        for (int i = 0; i < n; i += 1) {
            s[i] ^= 1;
        }
        for (int i = 0; i < m; i += 1) {
            t[i] ^= 1;
        }
    }
    if (m == 1) {
        for (int i = 0; i < n; i += 1) {
            if (s[i] == 0) {
                return -1;
            }
        }
        return 0;
    }
    if (m == 2) {
        if (t[0] != t[1]) {
            int count = 0;
            for (int i = 0; i + 1 < n; i += 1) {
                if (s[i] == 0 && s[i + 1] == 1) {
                    count += 1;
                }
            }
            return count;
        } else {
        }
    }
    return -1;
}

int32_t main() {
    if (1) {
        ios::sync_with_stdio(0);
        cin.tie(0);
        cout.tie(0);
    }
    string ss, ts;
    cin >> ss >> ts;
    n = ss.size();
    m = ts.size();
    s.resize(n);
    for (int i = 0; i < n; i += 1) {
        s[i] = (ss[i] - '0');
    }
    t.resize(m);
    for (int i = 0; i < m; i += 1) {
        t[i] = (ts[i] - '0');
    }
    cout << solve() << "\n";
    return 0;
}