CF466A.Cheap Travel

Ann has recently started commuting by subway. We know that a one ride subway ticket costs $a$ rubles. Besides, Ann found out that she can buy a special ticket for $m$ rides (she can buy it several times). It costs $b$ rubles. Ann did the math; she will need to use subway $n$ times. Help Ann, tell her what is the minimum sum of money she will have to spend to make $n$ rides?

The single line contains four space-separated integers $n$ , $m$ , $a$ , $b$ ( $1<=n,m,a,b<=1000$ ) — the number of rides Ann has planned, the number of rides covered by the $m$ ride ticket, the price of a one ride ticket and the price of an $m$ ride ticket.

Print a single integer — the minimum sum in rubles that Ann will need to spend.

In the first sample one of the optimal solutions is: each time buy a one ride ticket. There are other optimal solutions. For example, buy three $m$ ride tickets.