学步园

简单贪心，trick蛮多的，调了许久~~~

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const int MAXN = 25010;
const int INF = 0x3f3f3f3f;

struct Node
{
    int l, r;
    Node() {}
    Node(int t_l, int t_r) : l(t_l), r(t_r) {}
    friend bool operator < (const Node &p1, const Node &p2)
    {
        if(p1.l == p2.l)
            return p1.r > p2.r;
        return p1.l < p2.l;
    }
}p[MAXN];
int N, T, total, ans;


int main()
{
    //freopen("aa.in", "r", stdin);
    //freopen("bb.out", "w", stdout);


    while(scanf("%d %d", &N, &T) != EOF)
    {
        for(int i = 0; i < N; ++i)
        {
            scanf("%d %d", &p[i].l, &p[i].r);
        }
        sort(p, p + N);
        if(p[0].l > 1)
        {
            printf("-1\n");
        }
        else
        {
            int id = 0;
            total = 0; ans = 0;
            bool flag = true;
            while(id < N)
            {
                total++;
                int t_max = -INF;
                flag = false;
                while(p[id].l <= total && id < N)
                {
                    t_max = max(t_max, p[id].r);
                    id++;
                    flag = true;
                }
                if(!flag)
                    break;
                total = t_max, ans++;
                if(total >= T) break;
            }
            if(!flag || total < T)
                printf("-1\n");
            else
                printf("%d\n", ans);
        }
    }

    return 0;
}