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

if __name__ == "__main__":
    main()