#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; }