#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctime>
#include<cctype>
#include<climits>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<memory>
#include<functional>
#include<set>
 
 using namespace std;
  
  #define ALL(g) (g).begin(),(g).end()
  #define REP(i, x, n) for(int i = x; i < n; i++)
  #define rep(i,n) REP(i,0,n)
  #define F(i,j,k) fill(i[0],i[0]+j*j,k)
  #define P(p) cout<<(p)<<endl;
  #define EXIST(s,e) ((s).find(e)!=(s).end())
  #define INF 1<<25
  #define pb push_back
   
   typedef vector<int> vi;
   typedef vector<long long> vl;
   typedef vector<double> vd;
   typedef pair<int,int> pii;
   typedef pair<long,long> pll;
   typedef long long ll;
int N,D;
int in[101][2],dp[101][2];//[day][place]
/*void dfs(int day,int yen,int place){
	if(day==N)
		return ;
	if(place==0)
		yen+=t[day];
	else
		yen+=k[day];
	dp[day][place]=max(
	//stay
	dfs(day+1,yen,place);
	//move
	dfs(day+1,yen,1-place);
}
*/
int main(){
	cin>>N>>D;
	rep(i,N)
		cin>>in[i][0]>>in[i][1];
	dp[0][1]=-D;
	rep(i,N){
		rep(j,2){
			//stay
			dp[i+1][j]=max(dp[i+1][j],dp[i][j]+in[i][j]);
			//move
			dp[i+1][1-j]=max(dp[i+1][1-j],dp[i][j]+in[i][1-j]-D);
		}
	}
	int ans=max(dp[N][0],dp[N][1]);
	cout<<ans<<endl;
	return 0;
}