int getTotalX(vector<int> a, vector<int> b) {
    // assume vectors are sorted and a_n <= b_1    
    int betweenCount = 0;
    
    for (int q = a.back(); q <= b.front(); q += a.back()) {
        bool isMultiple = std::all_of(a.begin(), a.end(),
            [q] (int el) { return q % el == 0; }); 
        bool isFactor = std::all_of(b.begin(), b.end(),
            [q] (int el) { return el % q == 0; });
        if (isMultiple && isFactor)
            betweenCount++;
    }
    
    return betweenCount;
}