#include<algorithm>
#include<cmath>
#include<cstdio>
#include<functional>
#include<iomanip>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<string>
#include<utility>
#include<vector>

using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
const ll mod = 1000000007;
#define rep(i,n) for(int i=0;i<n;i++)
#define repl(i,s,e) for(int i=s;i<e;i++)
#define reple(i,s,e) for(int i=s;i<=e;i++)
#define revrep(i,n) for(int i=n-1;i>=0;i--)
#define all(x) (x).begin(),(x).end()

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

	vector<P> costs;
	rep(i, N)
	{
		ll t, k;
		cin >> t >> k;
		costs.emplace_back(t, k);
	}

	vector<vector<ll>> dp(2, vector<ll>(costs.size(), 0));
	dp[0][0] = costs[0].first;
	dp[1][0] = costs[0].second - D;

	repl(i, 1, N)
	{
		dp[0][i] = max(dp[0][i - 1]     , dp[1][i - 1] - D) + costs[i].first;
		dp[1][i] = max(dp[0][i - 1] - D , dp[1][i - 1]    ) + costs[i].second;
	}

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

	return 0;
}