#include<bits/stdc++.h>
using namespace std;
int n;
int dp[20005][2],l[20005],r[20005];
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>l[i]>>r[i];
	}
	dp[1][0]=r[1]-1+r[1]-l[1];
	dp[1][1]=r[1]-1;
	for(int i=2;i<=n;i++){
		dp[i][0]=min(dp[i-1][0]+abs(l[i-1]-r[i])+1+r[i]-l[i],dp[i-1][1]+abs(r[i-1]-r[i])+1+r[i]-l[i]);
		dp[i][1]=min(dp[i-1][0]+abs(l[i-1]-l[i])+1+r[i]-l[i],dp[i-1][1]+abs(r[i-1]-l[i])+1+r[i]-l[i]);
	}
	int ans=min(dp[n][0]+n-l[n],dp[n][1]+n-l[n]);
	cout<<ans;
	return 0;
}