N, K, X, Y = map(int, input().split())
A = list(map(int, input().split()))
C = [(a - 2) // K + 1 for a in A]
C.sort()

mp = 0
recover = 0

for i in range(N):
    if (N - i) * X > Y:
        mp += (C[i] - recover) * Y
        recover = C[i]
    else:
        for j in range(N):
            mp += max(C[j] - recover, 0) * X
        break

print(mp)