long long kth_smallest(long long k){
long long lo = 0, hi = tree.size()-1;
long long mid = -1;
while(lo < hi){
mid = (lo + hi)/2;
if(k <= tree.get_sum(mid))
hi = mid;
else
lo = mid + 1;
}
//cerr << "sum up to " << lo << " is " << tree.get_sum(lo) << endl;
if(tree.get_sum(lo) < k) return -1;
return lo;
}
long long Rank(long long n){
return tree.get_sum(n-1);
}