import java.util.*;
public class main {
public static void main(String args[])
{
    Scanner s=new Scanner(System.in);
    int n=s.nextInt();
    int x=s.nextInt();
    int []start=new int[n];
    int []end=new int[n];
    for(int i=0;i<n;i++)
    {
        start[i]=s.nextInt();
        end[i]=s.nextInt();
    }
    long sum=0;
    for(int i=0;i<n;i++)
    {
        int diff=0;
        if(i==0)
        {
            diff=start[i]-1;
        }
        else
        {
            diff=start[i]-end[i-1]-1;
        }
        int rem=diff%x;
        int magic=end[i]-start[i]+1;
        sum=sum+rem+magic;


    }
    System.out.println(sum);
}
}