bzoj 2152

 /**************************************************************
     Problem: 2152
     User: idy002
     Language: C++
     Result: Accepted
     Time:500 ms
     Memory:2612 kb
 ****************************************************************/

 #include <cstdio>
 #define N 20010

 typedef long long dnt;

 int n;
 int head[N], dest[N+N], wght[N+N], next[N+N], ntot;
 int fat[N], vis[N], siz[N], bac[N], dis[N], siz_tot, cur_root;
 int vv[], vtot;
 dnt path_tot, path_ans;

 void insert( int u, int v, int w ) {
     ntot++;
     next[ntot] = head[u];
     wght[ntot] = w;
     dest[ntot] = v;
     head[u] = ntot;
 }

 void dfs_root( int u ) {
     siz[u] = , bac[u] = ;
     for( int t=head[u]; t; t=next[t] ) {
         int v=dest[t];
         if( vis[v] || v==fat[u] ) continue;
         fat[v] = u;
         dfs_root(v);
         siz[u]+=siz[v];
         if( siz[v]>bac[u] ) bac[u]=siz[v];
     }
     if( siz_tot-siz[u]>bac[u] ) bac[u]=siz_tot-siz[u];
     if( bac[cur_root]>bac[u] ) cur_root=u;
 }
 void dfs_read( int u ) {
     int a = (-dis[u]%)%;
     path_ans += vv[a];
     path_tot += vtot;
     for( int t=head[u]; t; t=next[t] ) {
         int v=dest[t], w=wght[t];
         if( vis[v] || v==fat[u] ) continue;
         dis[v]=dis[u]+w;
         fat[v]=u;
         dfs_read( v );
     }
 }
 void dfs_write( int u ) {
     int a = dis[u]%;
     vv[a]++;
     vtot++;
     for( int t=head[u]; t; t=next[t] ) {
         int v=dest[t];
         if( vis[v] || v==fat[u] ) continue;
         dfs_write(v);
     }
 }
 void bfs( int rt ) {
     siz_tot = siz[rt];
     cur_root = ;
     fat[rt] = rt;
     dfs_root(rt);

     rt=cur_root;
     vis[rt] = true;
     vv[] = ;
     vtot = ;
     for( int t=head[rt]; t; t=next[t] ) {
         int v=dest[t], w=wght[t];
         if( vis[v] ) continue;
         dis[v] = w;
         fat[v] = rt;
         dfs_read(v);
         dfs_write(v);
     }
     vv[] = vv[] = vv[] = ;
     vtot = ;
     for( int t=head[rt]; t; t=next[t] ) {
         int v=dest[t];
         if( vis[v] ) continue;
         bfs(v);
     }
 }
 void build_vdcp() {
     bac[] = n;
     siz[] = n;
     bfs(  );
 }
 dnt gcd( dnt a, dnt b ) {
     return b ? gcd(b,a%b) : a;
 }

 int main() {
     scanf( "%d", &n );
     for( int i=,u,v,w; i<n; i++ ) {
         scanf( "%d%d%d", &u, &v, &w );
         insert( u, v, w );
         insert( v, u, w );
     }
     build_vdcp();
     path_tot *= ;
     path_ans *= ;
     path_tot += n;
     path_ans += n;
     dnt cd = gcd(path_tot,path_ans);
     printf( "%lld/%lld\n", path_ans/cd, path_tot/cd );
 }