There is a hotel with the following accommodation fee:    * X yen (the currency of Japan) per night, for the first K nights   * Y yen per night, for the (K+1)-th and subsequent nights  Tak is staying at this hotel for N consecutive nights. Find his total accommodation fee.
*a,= [int(input(i)) for i in range(4)]
print(a[1]*a[2]+(a[0]-a[1])*a[3])
*a,= [int(input()) for i in range(4)]
print(a[1]*a[2]+(a[0]-a[1])*a[3] if a[0]>a[1] else a[0]*a[2])