#include <bits/stdc++.h>
#include <iomanip>
#pragma GCC optimize("Ofast,unroll-loops")
using namespace std;
ifstream fin("tunel.in");
ofstream fout("tunel.out");
struct ceva
{
    int it,cost;
}p,p1;
vector<ceva>v[300];
queue<ceva>q;
int n,m,a,b,c,i,cnt,ans;
int main()
{
    fin>>n>>m;
    for(i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        v[a].push_back({b,c});
        v[b].push_back({a,c});
    }
    q.push({1,0});
    while(cnt!=n*n*n/2-1)
    {
        p=q.front();
        q.pop();
        for(auto it:v[p.it])
            if(it.it==n)
            {
                cnt++;
                ans=ans+it.cost+p.cost;
            }
            else q.push({it.it,it.cost+p.cost});
    }
    if(cnt!=0)
        fout<<fixed<<setprecision(4)<<ans/cnt;
    else
        fout<<ans;
    return 0;
}