1793A-YetAnotherPromotion.cpp

#include <cstdio>
typedef long long ll;

int main(){

    ll t; scanf("%lld", &t);
    while(t--){
        ll a, b; scanf("%lld %lld", &a, &b);
        ll n, m; scanf("%lld %lld", &n, &m);
        ll cfa = m * a * (n / (m + 1));
        ll cfb = (m + 1) * b * (n / (m + 1));
        ll cf = (cfa < cfb) ? cfa : cfb;
        ll cs = (a < b ? a : b) * (n % (m + 1));
        ll cost = cf + cs;
        printf("%lld\n", cost);
    }

}