poj 2376 Cleaning Shifts

#include <cstdio>#include <algorithm>using namespace std;pair<int, int> v[25005];int main(){    int n, t; scanf("%d%d", &n, &t);    for(int i = 0; i < n; ++i)        scanf("%d%d", &v[i].first, &v[i].second);    sort(v, v + n);    int from = 1, to = 0, cnt = 1;    for(int i = 0; i < n; ++i){        if(v[i].first > from){ ++cnt; from = to + 1;        }        if(v[i].first <= from){ to = max(to, v[i].second); if(to >= t) break;        }        else break;    }    printf("%dn", to < t ? -1 : cnt);    return 0;}