#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_SIZE 10005
struct Edge {
    int src;
    int dest;
    int weight;
};
struct Edge e[MAX_SIZE];
int comp(const void* a, const void* b)
{
    struct Edge* a1 = (struct Edge*)a;
    struct Edge* b1 = (struct Edge*)b;
    return a1->weight > b1->weight;
}
int par[MAX_SIZE];
int rank2[MAX_SIZE];
void init(int n) {
    for (int i = 0; i < n; i++) {
        par[i] = i;
        rank2[i] = 0;
    }
}
int find(int x) {
    if(par[x] == x) {
        return x;
    } else {
        return par[x] = find(par[x]);//路径压缩
    }
}
void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if(x == y) {
        return;
    }
    if(rank2[x] < rank2[y]) {
        par[x] = y;
    } else {
        par[y] = x;
        if(rank2[x] == rank2[y]) {
            rank2[x]++;
        }
    }
    
}
bool same(int x, int y) {
    return find(x) == find(y);
}
int main() {
    int N, M;
    while(scanf("%d%d", &M, &N)) {
        if(M == 0) {
            return 0;
        }
        
        for(int m = 0; m < M; m++) {
            int s, d, w;
            scanf("%d%d%d", &s, &d, &w);
            e[m].src = s;
            e[m].dest = d;
            e[m].weight = w;
        }
        if(M < N - 1) {
            printf("?\n");
            continue;
        }
        //sort
        qsort(e, M, sizeof(struct Edge), comp);
        int ans = 0;
        int count = 0;
        init(N);
        for(int m = 0; m < M; m++) {
            if(same(e[m].src - 1, e[m].dest - 1)) {
                continue;
            }
            unite(e[m].src - 1, e[m].dest - 1);
            ans += e[m].weight;
            count++;
            if(count == N - 1) {
                break;
            }
        }
        int root = 0;
        for(int n = 0; n < N; n++) {
            if(par[n] == n) {
                root++;
            }
        }
        if(root > 1) {
            printf("?\n");
        } else {
            printf("%d\n", ans);
        }
    }
    return 0;
}

dpx[i][j] = min(dpx[i][j - 1], dpy[i][j - 1] + (matrix[i + 1][j - 1] != 'b')) + (matrix[i][j] == 'b');
dpy[i][j] = min(dpy[i - 1][j], dpx[i - 1][j] + (matrix[i - 1][j + 1] != 'b')) + (matrix[i][j] == 'b');