#include <iostream>

 #include <cstdio>

 #include <algorithm>

 #define N 110

 #define M 5000

 using namespace std;

 int n, m, u[M], v[M], w[M], r[M], p[N];

 bool cmp(int i, int j){ return w[i]<w[j]; }

 int f(int x){ return p[x] == x ? x : p[x] = f(p[x]); }

 int main()

 {

     while (cin >> m >> n&&m)

     {

         for (int i = ; i <= n; ++i) p[i] = i;

         for (int j = ; j <= m; ++j) r[j] = j;

         for (int j = ; j <= m; ++j) cin >> u[j] >> v[j] >> w[j];

         sort(r + , r +  + m, cmp);

         int ans = , k = ;

         for (int j = ; j <= m&&k<n - ; ++j)

         {

             int e = r[j]; int x = f(u[e]), y = f(v[e]);

             if (x != y)ans += w[e], ++k, p[x] = y;

         }

         if (k<n - )puts("?");

         else cout << ans << endl;

     }

     return ;

 }