#include<iostream>
#include<algorithm>
using namespace std;

int main() {
	int n, d;
	cin >> n >> d;
	int dp[110][2];//i日目にjで働く(0:Tokyo,1:Kyoto)
	//初期値の設定
	dp[0][0] = 0;dp[0][1] = 0;
	int t, k;
	cin >> t >> k;
	dp[1][0] = t;dp[1][1] = k - d;
	for (int i = 1;i < n;i++) {
		//漸化式の計算
		int t, k;
		cin >> t >> k;
		dp[i + 1][0] = max(dp[i][0], dp[i][1] - d) + t;
		dp[i + 1][1] = max(dp[i][0] - d, dp[i][1]) + k;
	}
	cout << max(dp[n][0], dp[n][1]) << endl;
}