n,a,k,b = map(int,input().split())
L = []
if n%2==0:
    #全手搬
    x = n*a
    #全车搬
    if n%k == 0:
        y = n//k*b
    else:
        y = (n//k+1)*b
    #手+车结合
    for i in range(1,n): #一块儿转
        z = 0
        j = n-i #放车上的转
        if j%k==0:
            z += j//k*b+i*a
        else:
            z += (j//k+1)*b+i*a
        L.append(z)
    w = min(L)
    print(min(x,y,w))
else:
    print(-1)