n,k,x,y = map(int,input().split())
a = [int(i)-1 for i in input().split()]

ali = [0]*(n+1)

for i in range(n):
    ali[i+1] = (a[i]+k-1)//k

ali.sort()
ans = 0
idx = -1
for i in range(n):
    if (n-i) * x > y:
        idx = i
        continue
    else:
        ans += (ali[i+1]-ali[idx+1])*x
ans += ali[idx+1]*y
print(ans)