#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>
#include<iterator>

#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define rep(i,n) for(int i=0; i<n; i++)
#define INF (1<<20)

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ll, char> plc;


int main()
{
	ll n, d;
	ll dp[110][2],t[110],k[110];
	cin >> n >> d;

	rep(i, n)cin >> t[i] >> k[i];
	dp[0][1] = t[0];
	dp[0][0] = k[0] - d;

	for (int i = 1; i < n; i++) {
		dp[i][1] = t[i] + max(dp[i-1][1], dp[i - 1][0] - d);
		dp[i][0] = k[i] + max(dp[i-1][1]-d, dp[i-1][0]);
	}
	cout << max(dp[n-1][1],dp[n-1][0]) << endl;
	return	0;
}