#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()

int n,d;
vector<int> t(100),k(100);

int memo[100][2]={};
bool done[100][2]={};

int Search(int day,bool tokyo){
    if(day==n){
        return 0;
    }
    if(done[day][tokyo==true?1:0]==true){
        return memo[day][tokyo==true?1:0];
    }
    done[day][tokyo==true?1:0]=true;
    if(tokyo==true){
        return memo[day][1]=max(Search(day+1,1)+t[day],Search(day+1,0)+k[day]-d);
    }else{
        return memo[day][0]=max(Search(day+1,0)+k[day],Search(day+1,1)+t[day]-d);
    }
}

int main(){
    cin>>n>>d;
    REP(i,n){
        cin>>t[i]>>k[i];
    }
    cout<<Search(0,true)<<endl;
    return 0;
}