#include<iostream>
using namespace std;

typedef long long ll;
int main(){
  int n,D;
  cin>>n>>D;
  ll T,K,ans=0;
  bool f=true;//true:tokyo false:kyoto
  for(int i=0;i<n;i++){
    cin>>T>>K;
    if(f){
      if(T>=K-D) ans+=T;else{f=false; ans+=K-D;}
    }else{
      if(T-D>K){ ans+=T-D; f=true;}else{ans+=K;}
    }
  }
  cout<<ans<<"\n";

}