#include <iostream>
#define TYPE shift

using namespace std;

class shift {
	public :
	int start;
	int end;
	bool operator>= (shift s) {
		if (start > s.start || (start == s.start && end >=s.end)) {
			return true;
		}
		else 
			return false;
	}
	
	bool operator<= (shift s) {
		if (start < s.start || (start == s.start && end <=s.end)) {
			return true;
		}
		else
			return false;
	}
	
	void operator= (shift s) {
		start = s.start;
		end = s.end;
	}
};

shift shifts[25001];

// Quit sorting

int Partition(TYPE list[], int low, int high) {
	TYPE pivotkey;
	list[0] = list[low];
	pivotkey = list[low];
	while (low < high) {
		while (low < high && list[high] >= pivotkey)
			--high;
		list[low] = list[high];
		while (low < high && list[low] <= pivotkey)
			++ low;
		list[high] = list[low];
	}
	list[low] = list[0];
	return low;
}

void QuitSort(TYPE list[], int low, int high) {
	int pivotloc;
	if (low < high) {
		pivotloc = Partition(list, low, high);
		QuitSort(list, low, pivotloc - 1);
		QuitSort(list, pivotloc + 1, high);
	}
}

int main() {
	
	int n, t, i, j, count = 0;
	
	cin >> n >> t;
	
	for (i=1; i<=n; i++) {
		// cin >> shifts[i].start >> shifts[i].end;
		scanf("%d%d", &shifts[i].start, &shifts[i].end);
	}
	
	QuitSort(shifts, 1, n);
/*
	for (i=1; i<=n; i++) {
		cout << shifts[i].start << " " << shifts[i].end << endl;
	}
	*/

	int end = 0;
	int newend = 0;
	
	if (shifts[1].start != 1) {
		cout << "-1";
		return 0;
	}
	else {
		count++;
		for (i=1; i<=n; i++) {
			if (shifts[i].start == 1) {
				if (shifts[i].end == t) {
					cout << count;
					return 0;
				}
				end = shifts[i].end;
			}
			else
				break;
		}
	}
	
	for (; i<=n; i++) {
		if (shifts[i].start <= end + 1) {
			if (newend < shifts[i].end) {
				newend = shifts[i].end;
			}
		}
		else {
			if (newend == 0) {
				cout << "-1";
				return 0;
			}
			else {
				count ++;
				
				if (newend == t) {
					cout <<  count;
					return 0;
				}
				end = newend + 1;
				//cout << "!!!" << end <<endl;
				newend = 0;
				i--;
			}
		}
	}
	
	if (newend == t) {
		cout << (count + 1);
	}
	else {
		cout << "-1";
	}
	
	return 0;
}

• 这么长。。。。。 (2523) Lucifer 2005-11-01 02:04:43