#include <bits/stdc++.h>
using namespace std;
#define ll long long
 
ll n,m,x,y,b[3000000],c=0,t;
vector<pair<ll,ll>>a;
int main() {
	cin>>n>>m;
	for(int i=1;i<=n;i++){
		cin>>x>>y;
		a.push_back(make_pair(x,y));
	}
	sort(a.begin(),a.end());
	for(int i=0;i<n;i++){
		if(a[i].second<a[i].first){
			b[c]=a[i].second;
			c++;
		}
	}
	sort(b,b+c);
	if(c!=0)t=2*a[n-1].first-b[0];
	for(int i=1;i<c;i++){
		t+=b[i]-b[i-1];
	}
	cout<<t+(m-b[c-1]);
	return 0;
}

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MiA4NTM2MDgwNzUKMTAwNTg2NjgwIDIyNjA5NjU5NgoxNzcxMTA4NzAgMzU0ODg2MjY0

2 853608075
100586680 226096596
177110870 354886264