def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    K = int(input[idx])
    idx += 1
    
    A = list(map(int, input[idx:idx+N]))
    idx += N
    B = list(map(int, input[idx:idx+N]))
    idx += N
    
    if N == 1:
        print(0)
        return
    
    prev_prev = 0  # S[1]
    prev = prev_prev + A[0] + B[1]  # S[2]
    max_time = max(prev_prev, prev)
    
    for j in range(3, N+1):
        t1 = prev + A[j-2] + B[j-1]
        t2 = prev_prev + A[j-3] + B[j-1]
        current = min(t1, t2 + K)
        if current > max_time:
            max_time = current
        prev_prev, prev = prev, current
    
    print(max_time)

if __name__ == "__main__":
    main()