def chmax(DP,i,v):
    if DP[i] < v: DP[i] = v
def chmin(DP,i,v):
    if DP[i] > v: DP[i] = v

def main():
    N, D = map(int, input().split())
    dp = [[-1] * (2) for _ in range(N + 1)]
    dp[0][0] = 0

    for i in range(N):
        TK = list(map(int, input().split()))
        for j in range(2):
            if dp[i][j] == -1:
                continue
            # 移動なし
            chmax(dp[i + 1], j, dp[i][j] + TK[j])
            # 移動あり
            chmax(dp[i + 1], 1 - j, dp[i][j] + TK[1 - j] - D)

    print(max(dp[-1]))

if __name__ == "__main__":
    main()