求调特判

//离散化 
vector<int> lsh;
for(int i = 1;i <= n;i++) {
    lsh.push_back(a[i].l), lsh.push_back(a[i].r);
}
sort(lsh.begin(), lsh.end());
lsh.erase(unique(lsh.begin(), lsh.end()));
int len = lsh.size() + 1;
for(int i = 1;i <= n;i++) {
    a[i].l = lower_bound(lsh.begin(), lsh.end(), a[i].l) - lsh.begin();
    a[i].r = lower_bound(lsh.begin(), lsh.end(), a[i].r) - lsh.begin();
}

sort(a + 1, a + n + 1, cmp);
int l = 1, r = 0;
int mn = 1e18;
while(l <= n) {
    while(r < l || (r < n && T.range_query(1, 1, len, 1, len) < m)) { 
        //重合不足m,还没选够,右扩r
        r++;
        T.range_add(1, 1, len, a[r].l, a[r].r, 1);
    }   
    if(T.range_query(1, 1, len, 1, len) >= m)
        mn = min(mn, a[r].len - a[l].len);
    //收缩l
    T.range_add(1, 1, len, a[l].l, a[l].r, -1);//撤回l区间 
    l++;
}
cout << (mn == 1e18?-1:mn) << "\n";