#include <stdio.h>

int main() {
    int N,D;
    int A,B;
    scanf("%d %d",&N,&D);
    scanf("%d %d",&A,&B);
    int x = D*A;
    int y = N-D;
    int c = y*B;
    int t = x+c;
    printf("%d\n",t);

    return 0;
}