#include <bits/stdc++.h>
#define maxn 20005

using namespace std;

int n, c, m;
unsigned f[maxn][maxn];
vector<pair<int, int> > v[maxn];
vector<int> w;
int val[maxn];

void dfs(int i, int fa){
    for(auto it : v[i]){
        int to = it.first, co = it.second;
        if(to == fa) continue;
        val[to] = co;
        dfs(to, i);
    }
    unsigned sum = 0;
    for(auto it : v[i]){
        int to = it.first, co = it.second;
        if(to == fa) continue;
        unsigned mn = -1;
        for(int j = 0;j < m;j++) mn = min(mn, f[to][j]);
        sum += mn;
    }
    for(int j = val[i];j < m;j++){
        f[i][j] = 0;
        for(auto it : v[i]){
            int to = it.first;
            if(to == fa) continue;
            if(f[to][j] == -1){
                f[i][j] = -1;
                break;
            }
            f[i][j] += f[to][j];
        }
        f[i][j] = min(f[i][j], sum + c * (unsigned)v[i].size());
        if(i != 1) f[i][j] += w[j] - w[val[i]];
//		printf("%d %d %u--\n", i, j, f[i][j]);
    }
}

int main(){
    scanf("%d%d", &n, &c);
    for(int i = 1;i < n;i++){
        int x, y, z;
        scanf("%d%d%d", &x, &y, &z);
        v[x].push_back({y, z}), v[y].push_back({x, z});
        w.push_back(z);
    }
    if(n==1){
        printf("0\n");
        return 0;
    }
    sort(w.begin(), w.end());
    w.erase(unique(w.begin(), w.end()), w.end());
    m = w.size();
    for(int i = 1;i <= n;i++){
        for(auto &it : v[i]){
            it.second = lower_bound(w.begin(), w.end(), it.second) - w.begin();
        }
    }
    memset(f, -1, sizeof(f));
    dfs(1, 0);
    printf("%u", *min_element(f[1], f[1] + m));
}