def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    K = int(input[idx])
    idx += 1
    
    if N == 0:
        print(0)
        return
    
    A = list(map(int, input[idx:idx+N]))
    idx += N
    B = list(map(int, input[idx:idx+N]))
    idx += N
    
    A = [0] + A  # 1-based indexing
    B = [0] + B  # 1-based indexing
    
    t = [0] * (N + 1)
    t[1] = 0
    
    for j in range(2, N + 1):
        if j == 2:
            t_j = t[1] + A[1] + B[j]
        else:
            option1 = t[j-1] + A[j-1] + B[j]
            option2 = t[j-2] + A[j-2] + B[j] + K
            t_j = min(option1, option2)
        t[j] = t_j
    
    max_time = max(t[1:])
    print(max_time)

if __name__ == "__main__":
    main()