hdu 5310 Souvenir（BestCoder 1st Anniversary ($)）

#include<stdio.h>
#include<math.h>
#include<string.h>
#include<stdlib.h>
#define N 110

using namespace std;

int main()
{
    int t, sum, n, m, p, q, a, b;
    scanf("%d", &t);
    while(t--)
    {
        sum = 0;
        scanf("%d%d%d%d", &n, &m, &p, &q);
        a = n / m;
        b = n % m;
        if((n - b) * p > a * q)
            sum += a * q;
        else
            sum += (n - b) * p;
        if(b * p > q)
            sum += q;
        else
            sum += b * p;
        printf("%d\n", sum);
    }
    return 0;
}