#include<bits/stdc++.h>
using namespace std;
int main()
{
    freopen("jieniu.in","r",stdin);
    freopen("jieniu.out","w",stdout);
    long long a,b,c,d,f=0,n=0,m=0,g=0,l=0,j=0;
    long long s[10001],t[10001];
    cin>>a>>b;
     for(int y=0;y<a;)
    { y++;
       s[y]=0;}
    for(int y=0;y<a;)
    { y++;
       cin>>s[y]>>t[y]; 
          
    }
   
    for(int y=0;y<a;)
    {
        y++;
        n=s[y+1]-s[y];
       if(s[y]>b)
       n=0;
  if(y+1>a)
  n=b-s[y]+1;     
       if(s[y+1]>b)
       n=b-s[y]+1;
        m=m+t[y];
        
       
       
       if(n>=m)
       {
       g=g+m;
       m=0;}
       if(n<m)
       {
       g=g+n;                                  
        m=m-n;}
       
    }
    cout<<g;
    return 0;
 }