#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 1e6 + 10;

int n;
char s[N], t[N];

void solve() {
    scanf("%s%s", t + 1, s + 1);
    n = strlen(s + 1);

    int ans = 0;
    for (int i = 2; i < n; i ++ ) 
        if (s[i] != t[i] && s[i - 1] == s[i + 1] && s[i - 1] != s[i]) 
            ans ++ , s[i] ^= 1;

    if (strcmp(s + 1, t + 1)) ans = -1;
    printf("%d\n", ans);
}

int main() {
    int _; scanf("%d", &_);
    while (_ -- ) solve();
    return 0;
}