1 #include<iostream>//by chengdacaizi.
2 #include<stdio.h>
3 #define MAXN 25005
4 using namespace std;
5
6 struct Cows
7 {
8 int begin;
9 int end;
10 };
11
12 Cows cow[MAXN];
13
14 int compare(const void * i,const void * j);
15 int main()
16 {
17 //freopen("acm.acm","r",stdin);
18 int i;
19 int j;
20 int n;
21 int t;
22 bool boo;
23 int last;
24 int max;
25 int tem;
26 int max_num;
27 int result;
28 scanf("%d %d",&n,&t);
29 for(i = 0; i < n; ++ i)
30 {
31 scanf("%d %d",&cow[i].begin,&cow[i].end);
32 }
33 qsort(cow,n,sizeof(Cows),compare);
34 last = 1;
35 i = 0;
36 max = 0;
37 result = 0;
38 boo = true;
39 while(1)
40 {
41 max = -1;
42 while(cow[i].begin <= last)
43 {
44 if((tem = cow[i].end - last) > max)
45 {
46 max = tem;
47 max_num = i;
48 }
49 ++ i;
50 }
51 if(max == -1)
52 {
53 // cout<<cow[i].begin<<" 000000000000"<<endl;//很巧合的处理，当数组越界的时候，当然，还没有完成，那么自动跳出，cout<<-1；
54 boo = false;
55 break;
56 }
57 ++ result;
58 if(cow[max_num].end >= t)
59 {
60 break;
61 }
62 else
63 {
64 last = cow[max_num].end + 1;
65 }
66 }
67 if(boo)
68 cout<<result<<endl;
69 else
70 cout<<-1<<endl;
71
72 }
73
74 int compare(const void * i,const void * j)
75 {
76 return *((int *)i) - *((int *)j);
77 }
