N, K, X, Y = map(int, input().split())

A = list(map(lambda x: int(x) - 1, input().split()))

A.sort()

n = Y // X
if n < N:
    
    x = (A[N - n - 1] - 1) // K + 1
    
    ans = x * Y
    
    
    for i in range(N - n, N):
        
        ans += ((A[i] - 1) // K + 1 - x) * X
    
    print(ans)
    
else:
    
    print(sum([((a - 1) // K + 1) * x for a in A]))