//
// All truth are easy to understand once they are discovered.
// The point is to discover them
//
#include <bits/stdc++.h>
using namespace std;
const int N = 1e4 + 1;
vector<pair<int, int>> g[N];
vector<int> dp[N][2];
int sz[N], ndp[N + N][2];
// 0 - went backwards
// 1 - did not went backwards
void dfs(int v, int p) {
sz[v] = 1;
for (auto [u, c] : g[v]) {
if (u == p) continue;
dfs(u, v);
sz[v] += sz[u];
}
dp[v][0].resize(sz[v] + 1, 1e9);
dp[v][1].resize(sz[v] + 1, 1e9);
dp[v][0][1] = 0;
dp[v][1][1] = 0;
for (auto [u, c] : g[v]) {
if (u == p) continue;
for (int i = 0; i <= sz[v] + sz[u]; i++) {
ndp[i][0] = ndp[i][1] = 1e9;
}
for (int i = 0; i <= sz[v]; i++) {
for (int j = 1; j <= sz[u]; j++) {
ndp[i + j][0] = min(ndp[i + j][0], dp[v][0][i] + dp[u][0][j] + c * 2);
ndp[i + j][1] = min(ndp[i + j][1], dp[v][1][i] + dp[u][0][j] + c * 2);
ndp[i + j][1] = min(ndp[i + j][1], dp[v][0][i] + dp[u][1][j] + c);
}
}
for (int i = 0; i <= sz[v]; i++) {
dp[v][0][i] = min(dp[v][0][i], ndp[i][0]);
dp[v][1][i] = min(dp[v][1][i], ndp[i][1]);
}
}
}
void solve(){
int n, k, x;
cin >> n >> k >> x;
for (int i = 1, c, u, v; i < n; i++) {
cin >> u >> v >> c;
g[u].emplace_back(v, c);
g[v].emplace_back(u, c);
}
dfs(x, -1);
cout << dp[x][1][k];
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int tt = 1;
// cin >> tt;
while (tt--) {
solve();
cout << '\n';
}
return 0;
}
