#include <cstdio>
#include <vector>
#include <stack>
#include <algorithm>

#define MMAX 100007
#define PII pair<int, int>
#define pb push_back

using namespace std;
int n, m, x, y, z, sze, update;
double ans;
vector <PII> adj[MMAX];
PII tmp;
struct dfp
{
    int lung;
    int nod;
    int prob;
} aux, curr;
stack <dfp> dfs;

inline void compute_dfs()
{
    aux.lung = 0;
    aux.nod = 1;
    aux.prob = 1;
    dfs.push(aux);
    while(!dfs.empty())
    {
        curr = dfs.top();
        dfs.pop();
        if(curr.lung >= 1000) continue;
        if(curr.prob > 100000000) continue;
        if(curr.nod == n)
        {
            ans += ((double)curr.lung/(double)curr.prob);
            continue;
        }
        sze = adj[curr.nod].size();
        update = curr.prob * sze;
        for(int i = 0; i< sze; ++i)
        {
            aux.lung = curr.lung + adj[curr.nod][i].second;
            aux.nod = adj[curr.nod][i].first;
            aux.prob = update;
            dfs.push(aux);
        }
    }
}

int main()
{
    freopen("tunel.in", "r", stdin);
    freopen("tunel.out", "w", stdout);
    scanf("%d %d", &n, &m);
    for(; m; --m)
    {
        scanf("%d%d%d", &x, &y, &z);
        tmp.second = z;
        tmp.first = y;
        adj[x].pb(tmp);
        swap(tmp.first, x);
        adj[x].pb(tmp);
    }
    compute_dfs();
    printf("%lf\n", ans);
    return 0;
}