#include<cstdio>
#include<algorithm>
using namespace std;

struct vi
{
	int start;
	int end;
	int cost;
}edge[5000],et;

int p[101];

int Find(int n)
{
	return n==p[n]?n:p[n]=Find(p[n]);
}

bool cmp(vi a,vi b)
{
	return a.cost<b.cost;
}

int main()
{
	int n,m,i,k,sum,flag,ST,EN;
	while(scanf("%d%d",&n,&m)!=EOF,n)
	{
		for(i=1;i<=m;i++)
			p[i]=i;
		for(k=i=0;i<n;i++)
		{
			scanf("%d%d%d",&et.start,&et.end,&et.cost);
			edge[k++]=et;
		}
		sort(edge,edge+k,cmp);
		for(sum=i=0;i<k;i++)
		{
			ST=Find(edge[i].start);
			EN=Find(edge[i].end);
			if(ST!=EN)
			{
				p[ST]=EN;
				sum+=edge[i].cost;
			}
		}
		for(flag=0,i=1;i<=m;i++)
			if(p[i]==i)
				flag++;
		if(flag==1)
			printf("%d\n",sum);
		else
			puts("?");
	}
	return 0;
}
/**************************************************************
	Problem: 2204
	User: admin
	Language: C++
	Result: Accepted
	Time:12 ms
	Memory:1208 kb
****************************************************************/