#include <iostream>
#include <algorithm>
#include <cstring>

using namespace std;

int const INF = -1;

bool isTokyo = true;
int N, D, T, K;
int dp[110];

int main()
{
	cin >> N >> D;

	memset(dp, INF, sizeof(dp));

	dp[0] = 0;

	for (int i = 0; i < N; i++)
	{
		cin >> T >> K;
		if (isTokyo)
		{
			dp[i + 1] = max(dp[i] + T, dp[i] + K - D);
			isTokyo = false;
		}
		else
		{
			dp[i + 1] = max(dp[i] + T - D, dp[i] + K);
			isTokyo = true;
		}
	}

	cout << dp[N] << endl;
}