#include<bits/stdc++.h>
using namespace std;
int n,l;
long long ans=0;
struct mo{
	int start;
	int end;
}a[10005];
int main(){
	cin>>n>>l;
	for(int i=1;i<=n;i++){
		cin>>a[i].start>>a[i].end;
		ans+=a[i].end-a[i].start;
	}
	if(ans%l==0){
		cout<<ans/l<<endl;
		return 0;
	}
	else cout<<ans/l+1<<endl;
	return 0;
}