10874

#include <iostream>
#include <cstdio>
#include <queue>
#include <sstream>
#include <string>
#include <cstring>
#include <cmath>

#define ll long long
#define INF 500000000

using namespace std;

struct point{
	int left;
	int right;
};
point arr[20005];
ll dp[20005][2];

void init()
{
	for(int i=0;i<20005;i++)
		for(int j=0;j<2;j++)
			dp[i][j]=INF;
}

ll _min(ll a,ll b)
{
	if(a<b)
	return a;
	return b;
}
int main()
{
	int n;
	ll path;
	int y1,y2;
	while(scanf("%d",&n),n)
	{
		init();
		for(int i=1;i<=n;i++)
		{
			scanf("%d%d",&y1,&y2);
			arr[i].left=(y1<=y2?y1:y2);
			arr[i].right=(y1>=y2?y1:y2);
		}
		dp[1][0]=_min(dp[1][0], (arr[1].right-1) + (arr[1].right-arr[1].left) );
		dp[1][1]=_min(dp[1][1], (arr[1].right-1) );

		for(int i=2;i<=n;i++)
		{
			dp[i][0] = _min( dp[i][0] , _min( dp[i-1][0] + (arr[i].right - arr[i].left) + abs(arr[i].right-arr[i-1].left),
											  dp[i-1][1] + (arr[i].right - arr[i].left) + abs(arr[i].right-arr[i-1].right) ) )+1;
			dp[i][1] = _min( dp[i][1] , _min( dp[i-1][0] + (arr[i].right - arr[i].left) + abs(arr[i].left-arr[i-1].left),
											  dp[i-1][1] + (arr[i].right - arr[i].left) + abs(arr[i].left-arr[i-1].right)  ) )+1;
		}
		path = ( (dp[n][0]+n-arr[n].left) < (dp[n][1]+n-arr[n].right)? (dp[n][0]+n-arr[n].left):(dp[n][1]+n-arr[n].right));
		printf("%lld\n",path);
	}
	return 0;
}