#include <cstdio>
int main() {
int N, A, X, Y, hasil, hasil1, hasil2;
scanf("%d %d %d %d",&N,&A,&X,&Y);
if (A < N) {
hasil2 = N - A;
hasil1 = A * X;
hasil = hasil1 + hasil2 * Y;
printf("%d", hasil);
} else {
hasil = A * X;
printf("%d", hasil);
}}
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