#include <iostream>

#include <cstdio>

#include <cstring>

#include <queue>

using namespace std;

int n, m, cnt, ct, head[], vis[];

struct Edge {

    int s, t, v, next;

    friend bool operator < (Edge a, Edge b) {

        return a.v > b.v;

    }

}e[];

void add(int s, int t, int v) {

    e[cnt].t = t; e[cnt].v = v; e[cnt].next = head[s]; head[s] = cnt++;

}

int Prim() {

    int res = ;

    priority_queue <Edge> q;

    vis[] = ;

    ct++;

    for(int i = head[]; i != -; i = e[i].next) {

        q.push(e[i]);

    }

    while(!q.empty()) {

        Edge e1 = q.top();

        q.pop();

        vis[e1.t] = ;

        res += e1.v;

        ct++;

        if(ct == m) return res;

        for(int i = head[e1.t]; i != -; i = e[i].next) {

            if(!vis[e[i].t]) q.push(e[i]);

        }

    }

    return -;

} 

int main() {

    while(~scanf("%d%d", &n, &m), n) {

        cnt = ct = ;

        memset(head, -, sizeof(head));

        memset(vis, , sizeof(vis));

        for(int i = ; i < n; i++) {

            int s, t, v;

            scanf("%d%d%d", &s, &t, &v);

            add(s, t, v);

        }

        int ans = Prim();

        if(ans == -) puts("?");

        else printf("%d\n", ans);

    }

    return ;

}