mod = 1000000007
eps = 10**-9


def main():
    import sys
    input = sys.stdin.readline

    N, K = map(int, input().split())
    A = list(map(int, input().split()))
    B = list(map(int, input().split()))

    T = [0] * N
    for i in range(1, N):
        if i == 1:
            T[1] = A[0] + B[1]
        else:
            T[i] = min(T[i-1] + A[i-1] + B[i], T[i-2] + A[i-2] + B[i] + K)
    print(T[-1])


if __name__ == '__main__':
    main()