学习了

> #include <iostream>
> #include <cstdio>
> #include <algorithm>
> using namespace std;
> int N, T, M[1000050];
> int main()
> {
> 	cin >> N >> T;int a, b;
> 	for (int i = 0; i < N; i++) { scanf("%d%d", &a, &b); if (M[a] == 0)M[a] = b; else M[a] = max(M[a], b); }
> 	int nows = 1, cnt = 0, t, pre = 1, i;
> 	while (nows - 1 < T)
> 	{
> 		t = 0; i = pre;
> 		for ( ; i <= nows; i++)
> 			if (M[i] > t) { t = M[i]; pre = i; }
> 		if (t + 1 == nows) { cout << -1 << endl; return 0; }
> 		nows = t + 1;cnt++;
> 	}
> 	cout << cnt << endl;
> 	return 0;
> }