#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define endl "\n"
#define vec vector<ll>
#define vecvec vector<vector<ll>>
using namespace std;
/*#define FileName ""
string Ghhhh = ".in";
string Ghhhhh = ".out";
ifstream Girdi(FileName + Ghhhh);
ofstream Cikti(FileName + Ghhhhh);
#define cin Girdi
#define cout Cikti*/
const ll INF = 1e15;
ll n,k,x;
vector<vector<pair<ll,ll>>> adj;
vector<vector<vector<ll>>> dp;
map<pair<ll,ll>,ll> mapp;
inline vector<ll> comp(vector<ll> vec1, vector<ll> vec2){
vector<ll> ans(vec1.size());
for(ll i = 0 ; i < vec1.size() ; i++){
ans[i] = min(vec1[i],vec2[i]);
}
return ans;
}
inline vector<ll> total_min(vector<ll> a, vector<ll> b, ll cost,bool type){
vector<ll> combined(a.size() + b.size() - 1, INF);
combined[0] = 0;
for(ll i = 1 ; i < a.size() ; i++){
for(ll j = 0 ; j < b.size() ; j++){
if(j == 0){
combined[i+j] = min(combined[i+j],a[i]);
}else{
combined[i+j] = min(combined[i+j], a[i] + b[j] + (type ? cost * 2 : cost));
}
}
}
return combined;
}
inline void process(ll room, ll prev_room){
dp[room][0] = {0,0};
dp[room][1] = {0,0};
for(auto go : adj[room]){
if(prev_room == go.first) continue;
process(go.first,room);
vector<ll> vec1 = total_min(dp[room][1],dp[go.first][0],go.second,0);
vector<ll> vec2 = total_min(dp[room][0],dp[go.first][1],go.second,1);
dp[room][0] = comp(vec1,vec2);
dp[room][1] = total_min(dp[room][1],dp[go.first][1],go.second,1);
}
}
inline void solve(){
cin >> n >> k >> x;
adj.resize(n+1);
dp.resize(n+1,vector<vector<ll>>(2));
for(ll i = 1 ; i < n ; i++){
ll a,b,w;
cin >> a >> b >> w;
adj[a].pb({b,w});
adj[b].pb({a,w});
}
process(x,-1);
cout << dp[x][0][k] << endl;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
ll t = 1;
//cin >> t;
while(t--){
solve();
}
}
