#include <stdio.h>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <stack>
#include <ctype.h>
#define p(x,y) pair<x, y>
#define pii pair<int, int>
#define v(x) vector<x>
#define q(x) queue<x>
#define pq(x) priority_queue<x>
#define uppq(x, comp) priority_queue<x, vector<x>, comp>
#define st(x) set<x>
#define m(x, y) map<x, y>
#define fi(s,e) for(int i=s;i<e;i++)
#define fj(s,e) for(int j=s;j<e;j++)
#define fk(s,e) for(int k=s;k<e;k++)
typedef long long int ll;
typedef unsigned long long int ull;
typedef __int128 ulll;
using namespace std;
int n,need,x;
v(pii) pushvec;
v(v(pii)) vec;
int cnt[100010];
int dp[10010][10010][2];
//dp[start][visited rooms cnt][start==end?]
void find(int now, int parent){
dp[now][1][0]=0;
dp[now][1][1]=0;
fi(2, need+1) dp[now][i][0] = dp[now][i][1] = 999999999;
cnt[now]=1;
int sz = vec[now].size();
fi(0,sz){
pii next = vec[now][i];
if(next.first == parent) continue;
find(next.first, now);
for(int j = cnt[now]; j >= 1; j--){
fk(1, cnt[next.first]+1){
if(j + k > need) break;
dp[now][j+k][1] = min(dp[now][j+k][1], dp[now][j][1]+dp[next.first][k][1]+next.second*2);
dp[now][j+k][0] = min({dp[now][j+k][0], dp[next.first][k][1]+dp[now][j][0]+next.second*2, dp[now][j][1]+next.second+dp[next.first][k][0]});
}
}
cnt[now] += cnt[next.first];
}
return;
}
int main(void){
scanf("%d%d%d",&n,&need,&x);
fi(0,n+1) vec.push_back(pushvec);
fi(1,n){
int a,b,c;
scanf("%d%d%d",&a,&b,&c);
vec[a].push_back({b,c});
vec[b].push_back({a,c});
}
find(x, x);
printf("%d\n", dp[x][need][0]);
return 0;
}
