#include<bits/stdc++.h>
using namespace std;
const int N = 1e5+5;
long long a[N]={},b[N]={};
int main(){
    long long n,l,sum=0,m=0;
    cin>>n>>l;
    for(int i=1;i<=n;i++){
        cin>>a[i]>>b[i];
        m=(b[i]-a[i]);
        if(m%l==0){
        	sum+=m/l;
	    }
	    else{
		    sum+=m/l+1;
	    }
    }
    
    cout<<sum-1;
}