N, D = map(int, input().split())
DP = [[0] * (2) for _ in range(N + 1)]
DP[0][1] = -10**18

for i in range(1, N + 1):
    data = list(map(int, input().split()))
    
    for j in range(2):
        DP[i][j] = max(
            # 同じ場所から
            DP[i - 1][j] + data[j],
            # 場所移動あり
            DP[i - 1][1 - j] + data[j] - D
        )

ans = max(DP[-1])
print(ans)