/*
Source Code
Problem: 1001
Username: 2010201211
Run Time: 304MS
Memory: 1208K
Language:C++
JudgeStatus: Accepted
Author:MarkLiu
*/
#include <iostream>
#include <algorithm>
#include <cstdio>
#include <stdio.h>
#include <string.h>
using namespace std;const int N = 201;
const int INF = 1<<20;
int n,d[N][N],kd[N][N],m;
int g[N][N],gn[N];
double karp(){for(int i=0;i<=n;i++)for(int j=0;j<n;j++)kd[i][j]=INF;for(int i=0;i<n;i++)kd[0][i]=0;for(int i=1;i<=n;i++)for(int j=0;j<n;j++)if(kd[i-1][j]==INF) continue;else for(int k=gn[j]-1;k>=0;k--)kd[i][g[j][k]] = min(kd[i][g[j][k]],kd[i-1][j]+d[j][g[j][k]]);//cout << "debug" << endl;double minc=1e20;bool est=false;for(int i=0;i<n;i++){double mc=-1e20;if(kd[n][i]==INF) continue;for(int j=0;j<n;j++)if(kd[j][i]!=INF){mc=max(mc,1.0*(kd[n][i]-kd[j][i])/(n-j));est=true;}minc=min(minc,mc);}if(!est) minc=1e50;return minc;
}int main(){//freopen("in.txt","r",stdin);int a,b,c;while(scanf("%d%d",&n,&m)!=EOF){for(int i=0;i<n;i++)for(int j=0;j<n;j++){if(i==j) d[i][j]=INF;else d[i][j]=INF;}while(m--){scanf("%d%d%d",&a,&b,&c);if(c<d[a-1][b-1])d[a-1][b-1]=c;}memset(g,0,sizeof(g));memset(gn,0,sizeof(gn));for(int i=0;i<n;i++){int cnt=0;for(int j=0;j<n;j++){if(d[i][j]!=0 && d[i][j]!=INF){g[i][cnt++]=j;}}gn[i]=cnt;}/*for(int i=0;i<n;i++){for(int j=0;j<gn[i];j++){printf("%d ",g[i][j]);}cout << endl;}*/double res=karp();if(res==1e50) printf("INF\n");else{printf("%.3lf\n",res+1e-9);}}return 0;
}
/*
Sample Input:
3 3
1 2 2
2 3 2
3 1 1
8 7
1 1 14
8 1 969
5 3 518
3 5 948
7 2 962
8 2 221
1 1 935
3 3
2 3 2
3 2 2
1 1 1
Sample Output:
1.667
14.000
1.000
*/