#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;


int main() {
	int i, j;
	long long n, d;
	vector<long long>t, k;

	cin >> n >> d;

	for (i = 0; i < n; i++) {
		long long num1, num2;
		cin >> num1 >> num2;
		t.push_back(num1);
		k.push_back(num2);
	}

	vector<vector<long long>>dp(10000, vector<long long>(2, 0));	// [0][0]は東京  [0][1]は京都

	dp[0][0] = t[0];
	dp[0][1] = k[0] - d;


	for (i = 1; i < n; i++) {
		
		//dp[i][0] = max(dp[i - 1][0] + t[i], dp[i - 1][1] + k[i] - d);
		//dp[i][1] = max(dp[i - 1][1] + k[i], dp[i - 1][0] + t[i] - d);

		dp[i][0] = max(dp[i - 1][0] , dp[i - 1][1] - d) + t[i];
		dp[i][1] = max(dp[i - 1][1] , dp[i - 1][0] - d) + k[i];

	}



	cout << max(dp[n - 1][0], dp[n - 1][1]) << endl;






	getchar();
	getchar();
	return 0;
}