pii diam(int v, int pr = -1, int dist = 0) {
	int bg = v;
	for(auto it: gr[v]) {
		if(it != pr) {
			pii tmp = diam(it, v);
			if(tmp.fr > dist) {
				bg = tmp.sc;
				dist = tmp.fr;
			}
		}
	}
	return {dist + 1, bg};
}

main() {
  int u = diam(1).second
  int diamdist = diam(u).first // получаем дистанцию
}