// bzoj2878

#include<algorithm>

#include<iostream>

#include<cstdlib>

#include<cstring>

#include<cstdio>

#include<cmath>

#include<queue>

#define LL long long

#define inf (1ll<<30)

#define Pi acos(-1.0)

#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout)

using namespace std;

const int maxn=100010;

int n,m,cnt,head[maxn],fa[maxn],dag[maxn],son[maxn],a[maxn];

double ans,up[maxn],down[maxn];

struct edge {int to,next,w;}e[maxn<<1];

void link(int u,int v,int w) {

	e[++cnt]=(edge){v,head[u],w};head[u]=cnt;

	e[++cnt]=(edge){u,head[v],w};head[v]=cnt;

}

void topsort() {

	queue<int> q;

	for (int i=1;i<=n;i++) if (dag[i]==1) q.push(i);

	while (!q.empty()) {

		int x=q.front();q.pop();

		for (int i=head[x];i;i=e[i].next)

			if (dag[e[i].to]>1) if (--dag[e[i].to]==1) q.push(e[i].to);

	}

}

namespace Tree {

	void dfs(int x) {

		for (int i=head[x];i;i=e[i].next) if (dag[e[i].to]==1 && fa[x]!=e[i].to) {

				fa[e[i].to]=x;

				dfs(e[i].to);

				down[x]+=down[e[i].to]+e[i].w;

				son[x]++;

			}

		if (son[x]) down[x]/=son[x];

	}

	void dfs(int x,int len) {

		if (fa[x]) up[x]=(down[fa[x]]*son[fa[x]]-down[x]-len+dag[fa[x]]*up[fa[x]])/(son[fa[x]]-1+dag[fa[x]])+len;

		for (int i=head[x];i;i=e[i].next)

			if (dag[e[i].to]==1 && e[i].to!=fa[x]) dfs(e[i].to,e[i].w);

	}

	void main() {

		dag[1]=0;dfs(1);dfs(1,0);

		for (int i=1;i<=n;i++)

			ans+=(up[i]+down[i]*son[i])/(son[i]+(fa[i]!=0));

	}

}

namespace Circle {

	int rt,t=0;

	double dfs(int x,int fa) {

		double sum=0;int flag=0;

		for (int i=head[x];i;i=e[i].next)

			if (e[i].to!=fa && dag[e[i].to]==2 && e[i].to!=rt) sum+=e[i].w+dfs(e[i].to,x),flag=1;

		return x==rt ? sum/2 : (son[x]+flag ? (sum+down[x]*son[x])/(son[x]+flag) : 0);

	}

	void main() {

		for (int i=1;i<=n;i++) if (dag[i]==2) Tree::dfs(i),a[++t]=i;

		for (int i=1;i<=t;i++) up[rt=a[i]]=dfs(a[i],0);

		for (int i=1;i<=t;i++) Tree::dfs(a[i],0);

		for (int i=1;i<=n;i++)

			ans+=(up[i]*dag[i]+down[i]*son[i])/(son[i]+dag[i]);

	}

}

int main() {

	scanf("%d%d",&n,&m);

	for (int u,v,w,i=1;i<=m;i++) {

		scanf("%d%d%d",&u,&v,&w);

		link(u,v,w);dag[u]++,dag[v]++;

	}

	topsort();

	if (m==n-1) Tree::main();

	else Circle::main();

	printf("%.5lf",ans/n);

	return 0;

}