#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int N = 1e4 + 10;
vector<pair<int, int>> adj[N];
int n, k, x;
ll dp[2][N][N], knap[2][N];
int sz[N];
void dfs(int u, int p);
int main (int argc, char const *argv[]) {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> k >> x;
for(int i = 1, a, b, c; i < n; ++i) {
cin >> a >> b >> c;
adj[a].emplace_back(b, c);
adj[b].emplace_back(a, c);
}
dfs(x, 0);
cout << dp[1][x][k];
return 0;
}
void dfs(int u, int p){
sz[u] = 1;
for(auto [v, w] : adj[u]) {
if(v == p) continue;
dfs(v, u);
sz[u] += sz[v];
}
for(int i = 0; i <= sz[u]; ++i) knap[0][i] = knap[1][i] = 1e18;
knap[0][0] = knap[1][0] = 0;
int knapSize = 0;
for(auto [v, w] : adj[u]) {
if(v == p) continue;
for(int i = knapSize; i >= 0; --i) {
for(int j = 1; j <= sz[v]; ++j) {
knap[0][i + j] = min(knap[0][i + j], knap[0][i] + dp[0][v][j] + w*2);
knap[1][i + j] = min(knap[1][i + j], knap[0][i] + dp[1][v][j] + w);
knap[1][i + j] = min(knap[1][i + j], knap[1][i] + dp[0][v][j] + w*2);
}
}
knapSize += sz[v];
}
for (int i = 0; i < sz[u]; ++i) {
dp[0][u][i+1] = knap[0][i];
dp[1][u][i+1] = knap[1][i];
}
}
