1 #include <stdio.h>
 2 #include <string.h>
 3 #include <vector>
 4 #include <queue>
 5
 6 const int INF = 0x3f3f3f3f;
 7
 8 struct Edge
 9 {
10     int v, w;
11     Edge(){};
12     Edge(int v, int w)
13     {
14         this->v = v;
15         this->w = w;
16     }
17 };
18
19 std::vector<Edge>map[10010];
20 int dist[10010];
21 bool inque[10010];
22
23 void spfa(int s)
24 {
25     std::queue<int>q;
26     memset(inque, 0, sizeof(inque));
27     dist[s] = 0;
28     q.push(s);
29     inque[s] = 1;
30     while(!q.empty())
31     {
32         int u = q.front();
33         q.pop();
34         inque[u] = 0;
35         for(int i = 0; i < map[u].size(); i++)
36         {
37             int v = map[u][i].v;
38             if(dist[v] < dist[u] + map[u][i].w)
39             {
40                 dist[v] = dist[u] + map[u][i].w;
41                 if(!inque[v])
42                 {
43                     q.push(v);
44                     inque[v] = 1;
45                 }
46             }
47         }
48     }
49 }
50
51 int main()
52 {
53     int n;
54     int x, y;
55     while(scanf("%d", &n) != EOF)
56     {
57         for(int i = 0; i <= 10001; i++)
58         {
59             map[i].clear();
60         }
61         int low = INF, high = 0;
62         for(int i = 0; i < n; i++)
63         {
64             scanf("%d %d", &x, &y);
65             map[x].push_back(Edge(y+1, 2));
66             if(low > x)low = x;
67             if(high < y)high = y;
68         }
69         for(int i = low; i <= high+1; i++)
70         {
71             map[0].push_back(Edge(i, 0));
72             map[i].push_back(Edge(i+1, 0));
73             map[i+1].push_back(Edge(i, -1));
74             dist[i] = -INF;
75         }
76         map[0].push_back(Edge(high+1, 0));
77         spfa(low);
78         printf("%d\n", dist[high+1] - dist[low]);
79     }
80     return 0;
81 }