#include <bits/stdc++.h>


using namespace std;
typedef long long ll;
#define REP(i,n) for(int i=0; i<int(n); i++)
#define FOR(i,m,n) for(int i=int(m); i<int(n); i++)
#define ALL(obj) (obj).begin(),(obj).end()
#define VI vector<int>
#define VLL vector<long long>
#define VVI vector<vector<int>>
#define VVLL vector<vector<long long>>
#define VC vector<char>
#define VS vector<string>
#define VVC vector<vector<char>>
#define VB vector<bool>
#define VVB vector<vector<bool>>
#define fore(i,a) for(auto &i:a)

typedef pair <int, int> P;
template<class T> bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
const int  INF = 1 << 30;
const ll INFL = 1LL<<60;
const ll mod =1000000007;



int main() {
	
	ll n, d;
	cin >> n >> d;
	ll a = 0, b = 0;
	cin >> a >> b;
	b = max(0LL, b - d);
	REP(i, n - 1) {
		ll e, f;
		cin >> e >> f;
		ll g = max(a + e, b + e - d);
		b = max(b + f, a + f - d);
		a = g;
	}

	cout << max(a, b) << endl;

	
}