#include <bits/stdc++.h> using namespace std; #define GET_MACRO(_1, _2, _3, NAME, ...) NAME #define _repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++) #define _rep(i,n) _repl(i,0,n) #define rep(...) GET_MACRO(__VA_ARGS__, _repl, _rep)(__VA_ARGS__) #define mp(a,b) make_pair((a),(b)) #define pb(a) push_back((a)) #define all(x) (x).begin(),(x).end() #define uniq(x) sort(all(x)),(x).erase(unique(all(x)),end(x)) #define fi first #define se second #define dbg(...) _dbg(#__VA_ARGS__, __VA_ARGS__) void _dbg(string){cout<<endl;} template<class H,class... T> void _dbg(string s,H h,T... t){int l=s.find(',');cout<<s.substr(0,l)<<" = "<<h<<", ";_dbg(s.substr(l+1),t...);} template<class T,class U> ostream& operator<<(ostream &o, const pair<T,U> &p){o<<"("<<p.fi<<","<<p.se<<")";return o;} template<class T> ostream& operator<<(ostream &o, const vector<T> &v){o<<"[";for(T t:v){o<<t<<",";}o<<"]";return o;} #define INF 1120000000 int main(){ int n,d; cin>>n>>d; int v[2]; v[0] = 0; v[1] = -d; rep(i,n){ int a,b; cin>>a>>b; int u[2]; u[0] = max(v[0], v[1]-d)+a; u[1] = max(v[1], v[0]-d)+b; swap(v,u); } cout << max(v[0], v[1]) << endl; return 0; }