#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll n,t,last,ans;
int main(){
	cin>>n>>t;
	for(int i=1;i<=n;i++){
		ll d,b,s=0;
		cin>>d>>b;
		if(last>=t)break;
	    if(last>=d){
	    	if(d+(b-1)>=t){
	    		ans+=(t-last);
	    	}else{
	    		ans+=(d+(b-1)-last);
	    	}
		}else{
			if(d+(b-1)>=t){
				ans+=(t-d+1); 
			}else{
				ans+=b;
			}
		}
		if(d+(b-1)>last)last=d+(b-1);
	}
	cout<<ans<<endl;
	return 0;
}