#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e4 + 25;
const int inf = 1e9;
int n, k, x;
vector <pair <int, int>> adj[MAXN];
vector <int> dp[MAXN][2];
vector <int> minimize (vector <int> a, vector <int> b) {
vector <int> ret(max((int)a.size(), (int)b.size()));
for (auto &i : ret) i = inf;
for (int i = 0; i < (int)a.size(); i++) ret[i] = min(ret[i], a[i]);
for (int i = 0; i < (int)b.size(); i++) ret[i] = min(ret[i], b[i]);
return ret;
}
vector <int> add (vector <int> x, int a) {
for (auto &i : x) i += a;
return x;
}
vector <int> convolve (vector <int> a, vector <int> b) {
vector <int> ret((int)a.size() + (int)b.size() - 1);
for (auto &i : ret) i = inf;
for (int i = 0; i < (int)a.size(); i++) {
for (int j = 0; j < (int)b.size(); j++) {
ret[i + j] = min(ret[i + j], a[i] + b[j]);
}
}
return ret;
}
void dfs (int pos, int par) {
dp[pos][0] = {0, 0};
dp[pos][1] = {0, 0};
for (auto j : adj[pos]) {
if (j.first == par) continue;
dfs(j.first, pos);
auto a = dp[pos][0], b = dp[pos][1];
dp[pos][0] = minimize(a, convolve(a, add(dp[j.first][0], 2 * j.second)));
dp[pos][1] = minimize(b, minimize(convolve(b, add(dp[j.first][0], 2 * j.second)), convolve(a, add(dp[j.first][1], j.second))));
}
/*cout << pos << ": \n";
for (auto i : dp[pos][0]) cout << i << " ";
cout << '\n';
for (auto i : dp[pos][1]) cout << i << " ";
cout << '\n';
cout << '\n';*/
}
int main () {
ios::sync_with_stdio(0); cin.tie(0);
int n, k, x; cin >> n >> k >> x;
for (int i = 1; i < n; i++) {
int a, b, c; cin >> a >> b >> c;
adj[a].push_back({b, c});
adj[b].push_back({a, c});
}
dfs(x, -1);
cout << dp[x][1][k] << '\n';
}
