#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<string>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
#include<stack>
using namespace std;

#define REP(i,n) for(int i=0;(i)<(n);(i)++)
#define ll long long

int dp[102][2];

int main() {
	int N, D;
	int T[102], K[102];
	cin >> N >> D;

	REP(i, N) {
		cin >> T[i + 1] >> K[i + 1];
	}

	//0は東京，1は京都
	dp[0][0] = 0;
	dp[0][1] = -D;


	for (int i = 1;i <= N;i++) {
		dp[i][0] = max(dp[i - 1][0] + T[i], dp[i - 1][1] + T[i] - D);
		dp[i][1] = max(dp[i - 1][1] + K[i], dp[i - 1][0] + K[i] - D);
	}

	/*
	REP(i, N + 1) {
		cout << dp[i][0] << " " << dp[i][1] << endl;
	}
	*/
	
	cout << max(dp[N][0],dp[N][1]) << endl;
	return 0;
}