#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pll;
#define fastio ios::sync_with_stdio(false), cin.tie(0)
// #pragma GCC optimize("Ofast")
#define pb push_back
#define eb emplace_back
#define f first
#define s second
#define lowbit(x) x&-x
const int maxn = 2e5 + 5;
const ll INF = 1e18;
ll n, k, st;
vector<pll> adj[maxn];
vector<ll> dp[maxn][2];
vector<ll> total_min(vector<ll> a, vector<ll> b, int ded = 0){
vector<ll> nw(a.size() + b.size() - 1, INF);
for(int i = 0; i < a.size(); i++){
for(int j = 0; j < b.size(); j++){
nw[i + j] = min(nw[i + j], a[i] + b[j] - ded);
}
}
return nw;
}
void chgmin(vector<ll> &a, vector<ll> b){
int sz = max(a.size(), b.size());
a.resize(sz, INF);
b.resize(sz, INF);
for(int i = 0; i < sz; i++) a[i] = min(a[i], b[i]);
}
void dfs(int pos, int prev, int plen){
dp[pos][0] = {INF, plen};
dp[pos][1] = {INF, plen * 2};
for(auto [x, w] : adj[pos]){
if(x == prev) continue;
dfs(x, pos, w);
dp[pos][0] = total_min(dp[pos][0], dp[x][1]);
chgmin(dp[pos][0], total_min(dp[pos][1], dp[x][0], plen));
dp[pos][1] = total_min(dp[pos][1], dp[x][1]);
}
dp[pos][0][0] = dp[pos][1][0] = 0;
}
int main(void){
fastio;
cin>>n>>k>>st;
st--;
for(int i = 0; i < n - 1; i++){
int a, b, c;
cin>>a>>b>>c;
a--, b--;
adj[a].eb(b, c);
adj[b].eb(a, c);
}
dfs(st, -1, 0);
cout<<min(dp[st][0][k], dp[st][1][k])<<"\n";
}
