#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
#define el cout << '\n'
#define INF 0x3f3f3f3f
#define LINF 0x3f3f3f3f3f3f3f3f
#define N 10005
int n, k, r;
vector<pii> g[N];
vector<int> dp[N][2];
// dp(u, k, ok = 0/1) la tong do dai duong di ngan nhat di qua k dinh trong cay con goc u,
// ok = 0 la khong quay lai u va nguoc lai
inline void combineSubtree(vector<int> &Ans, vector<int> &a, vector<int> &b, int cost){
while(sz(Ans) < sz(a) + sz(b) - 1) Ans.emplace_back(INF);
for(int i=0; i<sz(a); ++i){
for(int j=0; j<sz(b); ++j){
Ans[i + j] = min(Ans[i + j], a[i] + b[j] + cost);
}
}
}
void dfs(int u, int p = -1){
dp[u][0] = dp[u][1] = {0, 0};
for(pii T:g[u]){
int v = T.fi, w = T.se;
if(v != p){
dfs(v, u);
vector<int> f[2];
for(int i=0; i<2; ++i) f[i] = dp[u][i];
combineSubtree(dp[u][0], f[0], dp[v][1], 2ll * w);
combineSubtree(dp[u][0], f[1], dp[v][0], w);
combineSubtree(dp[u][1], f[1], dp[v][1], 2ll * w);
}
}
}
void Solve(){
cin >> n >> k >> r;
for(int i=1; i<n; ++i){
int u, v, w; cin >> u >> v >> w;
g[u].emplace_back(v, w);
g[v].emplace_back(u, w);
}
dfs(r);
cout << dp[r][0][k];
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define NAME "TASK"
if(fopen(NAME".inp", "r")){
freopen(NAME".inp", "r", stdin);
freopen(NAME".out", "w", stdout);
}
Solve();
return 0;
}
