求最小值，即求最长路。设 s[i]为区间[0,i]被选中的数的个数，区间[a b]至少选2个数可以转化为:s[ b ] -s[a-1]>=2 -> s[a-1]+2 <= s[ b ] -> G[a-1].push_back({b, 2}), s[i]-s[i-1]<=1 -> s[i] -1 <=s[i-1] -> G[i].push_back({i-1, -1}), s[i]-s[i-1]>=0 -> s[i-1] <=s[i] -> G[i-1].push_back({i, 0}),

#include <bits/stdc++.h>
using namespace std;
const int N=1e4+10;
vector<pair<int,int>> G[N];
int st,ed,d[N],dd[N];bool v[N];
int spfa()
{
    memset(d,0,sizeof(d));
    memset(dd,0,sizeof(dd));
    memset(v,0,sizeof(v));
    deque<int> Q;
    for(int i=st;i<=ed;i++) Q.push_front(i),v[i]=1;
    d[st]=0;
    while(!Q.empty())
    {
        int x=Q.front();Q.pop_front();v[x]=0;
        for(auto i:G[x])
        {
            int y=i.first,w=i.second;
            if(d[y]<d[x]+w)
            {
                d[y]=d[x]+w;if(x==y){return -1;}//判断负自环 
                dd[y]=dd[x]+1;if(dd[y]>(ed-st+1)){return -1;}
                if(v[y]==0) Q.push_front(y),v[y]=1;
            }
        }
    }
    return d[ed]-d[st];
}
int main()
{
    int n;scanf("%d",&n);
    ed=0,st=N;
    for(int i=1,x,y;i<=n;i++)
    {
        scanf("%d%d",&x,&y);x++,y++;
        G[x-1].push_back({y,2});
        ed=max(ed,y);
        st=min(st,x-1);
    }
    for(int i=st+1;i<=ed;i++)
    {
        G[i].push_back({i-1,-1});
        G[i-1].push_back({i,0});
    }
    printf("%d\n",spfa());
    return 0;
}

/*
求最小值，即求最长路。
设 s[i]为区间[0,i]被选中的数的个数，
区间[a b]至少选2个数可以转化为:s[ b ] -s[a-1]>=2 -> s[a-1]+2 <= s[ b ] -> G[a-1].push_back({b, 2}),
s[i]-s[i-1]<=1 -> s[i]   -1 <=s[i-1] ->  G[i].push_back({i-1, -1}),
s[i]-s[i-1]>=0 -> s[i-1]    <=s[i]   ->  G[i-1].push_back({i, 0}),
*/
#include<bits/stdc++.h>
using namespace std;
const int N=1e4+10;
vector< pair<int,int> >G[N];
int st,ed,d[N],dd[N];bool v[N];
int spfa()
{
    memset(d,0,sizeof(d));
    memset(dd,0,sizeof(dd));
    memset(v,0,sizeof(v));
    deque<int>Q;
    for(int i=st;i<=ed;i++)Q.push_front(i),v[i]=1;
    d[st]=0;
    while(!Q.empty())
    {
        int x=Q.front();Q.pop_front();v[x]=0;
        for(auto i:G[x])
        {
            int y=i.first,w=i.second;
            if(d[y]<d[x]+w)
            {
                d[y]=d[x]+w;if(x==y){return -1;}//判断负自环 
                dd[y]=dd[x]+1;if(dd[y]>(ed-st+1)){return -1;}
                if(v[y]==0) Q.push_front(y),v[y]=1;
            }
        }
    }
    return d[ed]-d[st];
}
int main()
{
    int n;scanf("%d",&n);
    ed=0,st=N;
    for(int i=1,x,y;i<=n;i++)
    {
        scanf("%d%d",&x,&y);x++,y++;
        G[x-1].push_back({y,2});
        ed=max(ed,y);
        st=min(st,x-1);
    }
    for(int i=st+1;i<=ed;i++)
    {
        G[i].push_back({i-1,-1});
        G[i-1].push_back({i,0});
    }
    printf("%d\n",spfa());
    return 0;
}