#include<iostream>

#include<cstdio>

#include<cstdlib>

#include<cstring>

#include<cmath>

#include<map>

#include<set>

#include<vector>

#include<algorithm>

#include<stack>

#include<queue>

using namespace std;

#define INF 1000000000

#define eps 1e-8

#define pii pair<int,int>

#define LL long long int

int n,m,p[],num;

LL ans;

struct node

{

    int u,v;

    LL w;

} e[];

bool cmp(node aa,node bb)

{

    return aa.w<bb.w;

}

int finds(int x)

{

    return p[x]==x?x:p[x]=finds(p[x]);

}

int main()

{

   // freopen("in1.txt","r",stdin);

    //freopen("out.txt","w",stdout);

    while(scanf("%d%d",&n,&m)==)

    {

        if(n==) break;

        ans=;

        num=;

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

        {

            scanf("%d%d%I64d",&e[i].u,&e[i].v,&e[i].w);

        }

        for(int i=; i<=m; i++)

        {

            p[i]=i;

        }

        sort(e,e+n,cmp);

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

        {

            int x=finds(e[i].u);

            int y=finds(e[i].v);

            if(x!=y)

            {

                ans+=e[i].w;

                num++;

                p[x]=y;

            }

        }

        if(num==m-)

            printf("%I64d\n",ans);

        else

            printf("?\n");

    }

    //fclose(stdin);

    //fclose(stdout);

    return ;

}