def main():
    import sys
    input = sys.stdin.read().split()
    ptr = 0
    N = int(input[ptr])
    ptr += 1
    K = int(input[ptr])
    ptr += 1
    
    if N == 0:
        print(0)
        return
    
    A = list(map(int, input[ptr:ptr+N]))
    ptr += N
    B = list(map(int, input[ptr:ptr+N]))
    ptr += N
    
    if N == 1:
        print(0)
        return
    
    max_time = 0
    t_prev_prev = 0  # T[1]
    t_prev = t_prev_prev + A[0] + B[1]
    max_time = max(t_prev_prev, t_prev)
    
    for j in range(3, N + 1):
        a = t_prev + A[j-2] + B[j-1]
        b = t_prev_prev + A[j-3] + B[j-1] + K
        current = min(a, b)
        if current > max_time:
            max_time = current
        # Update for next iteration
        t_prev_prev, t_prev = t_prev, current
    
    print(max_time)

if __name__ == '__main__':
    main()