#include<bits/stdc++.h>
using namespace std;
int l,m,a[10002],b,c,s; 
int main(){
	//freopen("b.in","r",stdin);
	//freopen("b.out","w",stdout);
	cin>>l;
	for(int i=1;i<=l*2;i++){
		cin>>b>>c;
		a[b]=1;a[c]=1;
		for(int j=b;j<=c;j++){
			if(a[j]==0)
			a[j]=1;
		}
	}
	for(int i=1;i<=l*2;i++){
		if(a[i]!=0)
			s++;
	}
	cout<<s;
	//fclose(stdin);fclose(stdout);
	return 0;
}