C++ :

#include <bits/stdc++.h>
#define xx first
#define yy second
#define mp make_pair
#define pb push_back
#define fill( x, y ) memset( x, y, sizeof x )
#define copy( x, y ) memcpy( x, y, sizeof x )
using namespace std;

typedef long long LL;
typedef pair < int, int > pa;

const int MAXN = 15;
const int MAXM = 55;
const int MAXV = 1025;

int G[MAXN], bit[MAXV], cnt[MAXV], n, m;
long double f[MAXV][MAXM], g[MAXV][MAXM], C[MAXM][MAXM], ans;

inline int lowbit(int x) { return x & -x; }

int main()
{
#ifdef wxh010910
	freopen( "data.in", "r", stdin );
#endif
	scanf( "%d%d", &n, &m );
	for( int i = 1 ; i <= m ; i++ )
	{
		int x, y;
		scanf( "%d%d", &x, &y );
		x--; y--;
		G[ x ] |= 1 << y; G[ y ] |= 1 << x;
	}
	for( int i = 0 ; i <= m ; i++ )
	{
		C[ i ][ 0 ] = 1;
		for( int j = 1 ; j <= i ; j++ )
			C[ i ][ j ] = C[ i - 1 ][ j ] + C[ i - 1 ][ j - 1 ];
	}
	for( int S = 1, u ; S < ( 1 << n ) ; S++ )
	{
		bit[ S ] = __builtin_popcount( S );
		if( bit[ S ] == 1 ) { g[ S ][ 0 ] = 1; continue; }
		for( int i = 0 ; i < n ; i++ )
			if( ( S >> i ) & 1 )
				cnt[ S ] += bit[ S & G[ i ] ];
		cnt[ S ] >>= 1;
		u = lowbit( S );
		for( int T = S - 1 & S ; T ; T = T - 1 & S ) if( T & u )
			for( int i = 0 ; i <= cnt[ T ] ; i++ )
				for( int j = 0 ; j <= cnt[ S ^ T ] ; j++ )
					f[ S ][ i + j ] += g[ T ][ i ] * C[ cnt[ S ^ T ] ][ j ];
		for( int i = 0 ; i <= cnt[ S ] ; i++ )
			g[ S ][ i ] = C[ cnt[ S ] ][ i ] - f[ S ][ i ];
	}
	for( int i = 0 ; i <= m ; i++ ) 
		ans += f[ ( 1 << n ) - 1 ][ i ] / C[ m ][ i ];
	ans /= m + 1;
	printf( "%.6Lf\n", ans );
}