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Agent Johnny English has come across his next target, and he has to kill him. To make sure his clumsiness does not get in the way, he decides to fire exactly $n$ shots from his pistol. The pistol has a magazine that holds $m$ rounds. It starts empty.

Johnny can fully reload the pistol in $a$ seconds, or load one round into the magazine in $b$ seconds. Each shot takes exactly one second. Find the minimum time Johnny needs to fire exactly $n$ shots. He cannot fire an empty pistol, and he cannot load a round into a full magazine.

The first line contains four integers $n$, $m$, $a$, and $b$ ($1 \le n, m, a, b \le 10^4$). They are the number of shots to fire, the magazine size, the time for a full reload, and the time to load one round.

Print one integer, the minimum time Johnny needs to fire exactly $n$ shots.

In the test from the statement, Johnny fully reloads once, fires twice, loads one more round, and fires it.