onibus.cpp

#include <bits/stdc++.h>
using namespace std;

vector<vector<int>> adj;
vector<int> dist;

int bfs(int a, int b){

    dist[a] = 0;
    queue<int> q;
    q.push(a);

    while(!q.empty()){

        int x = q.front(); q.pop();

        for(int i=0; i<adj[x].size(); i++){

            int viz = adj[x][i];
            if(dist[viz] != -1) continue;

            dist[viz] = dist[x] + 1;
            q.push(viz);
        }
    }
    return dist[b];
}

int main(){

    int n,a,b; cin >> n >> a >> b;
    a--; b--;

    adj.resize(n+5);

    for(int i=1; i<n; i++){

        int p,q; 
        cin >> p >> q;
        p--;q--;
        adj[p].push_back(q);
        adj[q].push_back(p);
    }
    dist.assign(n,-1);

    cout << bfs(a,b) << endl;

}