N, K = map(int, input().split())
A = [int(a) for a in input().split()]
B = [int(a) for a in input().split()]
X = [0] * N
if N > 1: X[1] = A[0] + B[1]
for i in range(2, N): X[i] = min(X[i-2] + K + A[i-2] + B[i], X[i-1] + A[i-1] + B[i])
print(X[-1])