#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int nmax = 1e4 + 5;
int n, k, x, sz[nmax];
vector <pair <int, ll>> g[nmax];
ll dp[nmax][nmax][2], ndp[nmax][2];
void dfs(int u, int p) {
sz[u] = 1;
for (auto [v, w] : g[u]) {
if (v==p) continue;
dfs(v, u);
sz[u] += sz[v];
}
int cur = 1;
for (int i=2; i<=sz[u]; i++)
dp[u][i][0] = dp[u][i][1] = 1e15;
for (auto [v, w] : g[u]) {
if (v==p) continue;
for (int i=1; i<=cur+sz[v]; i++) {
ndp[i][0] = dp[u][i][0];
ndp[i][1] = dp[u][i][1];
}
for (int i=1; i<=cur; i++) {
for (int j=1; j<=sz[v]; j++) {
ndp[i + j][0] = min(ndp[i + j][0], dp[u][i][0] + dp[v][j][0] + 2 * w);
ndp[i + j][1] = min(ndp[i + j][1], dp[u][i][0] + w + dp[v][j][1]);
ndp[i + j][1] = min(ndp[i + j][1], dp[u][i][1] + dp[v][j][0] + 2 * w);
}
}
cur += sz[v];
for (int i=1; i<=cur; i++) {
dp[u][i][0] = ndp[i][0];
dp[u][i][1] = ndp[i][1];
}
}
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cin >> n >> k >> x;
for (int i=1; i<n; i++) {
int u, v, w;
cin >> u >> v >> w;
g[u].push_back({v, w});
g[v].push_back({u, w});
}
dfs(x, 0);
cout << min(dp[x][k][0], dp[x][k][1]);
return 0;
}
