#include<bits/stdc++.h>
using namespace std;
const int maxn=20010;
int dis(int a,int b){return abs(a-b);}
int n,f[maxn][2],l[maxn],r[maxn];
int main(){
	scanf("%d",&n);
	for(int i=1;i<=n;i++){
		scanf("%d%d",&l[i],&r[i]);
	}
	f[1][0]=dis(r[1],1)+dis(r[1],l[1]);
	f[1][1]=dis(r[1],1);
	for(int i=2;i<=n;i++){
		f[i][0]=min(dis(r[i],l[i-1])+f[i-1][0],dis(l[i],r[i-1])+f[i-1][1])+dis(l[i],r[i])+1;
		f[i][1]=min(dis(l[i],l[i-1])+f[i-1][0],dis(r[i-1],l[i])+f[i-1][1])+dis(l[i],r[i])+1;
	}
	printf("%d",min(f[n][0]+dis(n,l[n]),f[n][1]+dis(n,r[n])));
	return 0;
}

本人蒟蒻，勿喷，感谢