N, K, X, Y = map(int, input().split())
A = list(map(int, input().split()))
for i in range(N):
    A[i] -= 1
A.sort()
pos = 0
S = 0
HP = 0
while (N - pos) * X >= Y:
    S += ((A[pos] - HP) // K + ((A[pos] - HP) % K != 0)) * Y
    HP += ((A[pos] - HP) // K + ((A[pos] - HP) % K != 0)) * K
    pos += 1
    if pos == N:
        break
for j in range(pos, N):
    S += ((A[j] - HP) // K + ((A[j] - HP) % K != 0)) * X
print(S)