#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("avx2")
using namespace std;
using namespace __gnu_pbds;
#define fi first
#define se second
#define all(m) (m).begin(), (m).end()
#define rall(m) (m).rbegin(), (m).rend()
#define vec vector
#define sz(a) (int) (a).size()
#define mpp make_pair
#define mtt make_tuple
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef pair <ll, ll> pll;
typedef pair <int, int> pii;
typedef tuple <int, int, int> tui;
template <typename T>
using prq = priority_queue <T>;
template <typename T>
using pgq = priority_queue <T, vec <T>, greater <T>>;
template <typename T> bool umin(T &a, T b) { return a > b ? a = b, 1 : 0; }
template <typename T> bool umax(T &a, T b) { return a < b ? a = b, 1 : 0; }
const int inf = 1e9 + 42;
inline int solve(){
int n, k, x;
cin >> n >> k >> x, --x;
vec <vec <pii>> g(n);
for (int i = 0; i < n - 1; ++i){
int u, v, w;
cin >> u >> v >> w, --u, --v;
g[u].push_back({v, w});
g[v].push_back({u, w});
}
vec <vec <array <int, 2>>> dp(n);
auto mrg = [&](vec <array <int, 2>> a, vec <array <int, 2>> b, int len){
vec <array <int, 2>> c(sz(a) + sz(b) - 1, {inf, inf});
for (int i = 0; i < sz(a); ++i){
umin(c[i][0], a[i][0]);
umin(c[i][1], a[i][1]);
for (int j = 0; j < sz(b); ++j){
umin(c[i + j][0], a[i][0] + b[j][0] + 2 * len);
umin(c[i + j][1], min(a[i][1] + b[j][0] + 2 * len, a[i][0] + b[j][1] + len));
}
}
return c;
};
auto dfs = [&](auto &&dfs, int u, int p) -> void{
dp[u] = {{0, 0}, {0, 0}};
for (auto &[v, w]: g[u]) if (v != p){
dfs(dfs, v, u);
dp[u] = mrg(dp[u], dp[v], w);
}
}; dfs(dfs, x, x);
cout << dp[x][k][1] << "\n";
return 0;
}
inline void precalc(){}
signed main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int tst = 1; //cin >> tst;
precalc();
while(tst--) solve();
return 0;
}
