#include <iostream>

 #include <algorithm>

 #include <cstdio>

 #include <cstring>

 using namespace std;

 const int N = ;

 struct Edge {

     int t, nx, c;

     Edge() {}

     Edge(int t, int nx, int c) : t(t), nx(nx), c(c) {}

 } edge[N << ];

 int eh[N], ec;

 void init() {

     memset(eh, -, sizeof(eh));

     ec = ;

 }

 void addedge(int s, int t, int c) {

     edge[ec] = Edge(t, eh[s], c);

     eh[s] = ec++;

 }

 int q[N << ], dis[N];

 bool vis[N];

 int spfa(int s, int t) {

     memset(vis, , sizeof(vis));

     memset(dis, , sizeof(dis));

     int qh, qt, cur;

     qh = qt = ;

     q[qt++] = s;

     vis[s] = true;

     dis[s] = ;

     while (qh < qt) {

         //cout << cur << endl;

         cur = q[qh++];

         vis[cur] = false;

         for (int i = eh[cur]; ~i; i = edge[i].nx) {

             //cout << i << endl;

             Edge &e = edge[i];

             if (dis[e.t] > dis[cur] + e.c) {

                 dis[e.t] = dis[cur] + e.c;

                 q[qt++] = e.t;

                 vis[e.t] = true;

             }

         }

     }

     return dis[t];

 }

 int main() {

     int n, x, y;

     while (~scanf("%d", &n)) {

         init();

         int mn = , mx = -;

         for (int i = ; i < n; i++) {

             scanf("%d%d", &x, &y);

             addedge(y + , x, -);

             mn = min(mn, x);

             mx = max(mx, y + );

         }

         for (int i = ; i <= ; i++) {

             addedge(i, i + , );

             addedge(i + , i, );

         }

         printf("%d\n", -spfa(mx, mn));

     }

     return ;

 }