#include <set>
#include <bitset>
#include <list>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <string>
#include <vector>
#include <ios>
#include <iostream>
#include <fstream>
#include <sstream>
#include <iomanip>
#include <algorithm>
#include <utility>
#include <complex>
#include <numeric>
#include <functional>
#include <cmath>
#include <ctime>
#include <climits>
#include <cstdarg>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cctype>
#include <cassert>
#define lson rt << 1, l, m
#define rson rt << 1 | 1, m + 1, r
using namespace std;
const int INF = 0x7FFFFFFF;
const int MOD = 1000000000 + 7;
const double EPS = 1e-10;
const double PI = 2 * acos (0.0);
const int maxn = 10000 + 66;
struct Node
{
int u, v, w;
};
Node ans[maxn];
int n, m;
bool cmp (Node a, Node b)
{
return a.w < b.w;
}
int parent[maxn];
int UnFind (int val)
{
if (parent[val] == val)
return val;
return parent[val] = UnFind (parent[val]);
}
int kruskal ()
{
int remain = m, tot = 0;
for (int i = 0; i < n && remain > 1; i++)
{
int a = UnFind (ans[i].u), b = UnFind (ans[i].v);
if (a != b)
{
parent[a] = b;
tot += ans[i].w;
remain--;
}
}
if (remain > 1)
return -1;
return tot;
}
int main()
{
// freopen ("aaa.txt", "r", stdin);
while (scanf ("%d%d", &n, &m) != EOF)
{
if (n == 0)
break;
for (int i = 0; i <= m;i++)
parent[i] = i;
;
for (int i = 0; i < n; i++)
scanf ("%d%d%d", &ans[i].u, &ans[i].v, &ans[i].w);
sort (ans, ans + n, cmp);
int temp = kruskal ();
if (temp == -1)
cout << "?" << endl;
else
cout << temp << endl;
}
return 0;
}