#include<bits/stdc++.h>
using namespace std;long long q[33][305],A=0,all;int p[7],a[7],n,c,g;
inline void S(){int i,j,k,t=(1<<(n-1))-1,o,l;for(i=0;i<g;i++){if(i%n==0)continue;o=p[i%n];l=min(g,a[o]*n+i);o=1<<(o-1);for(j=t;j>=0;j--)if(!(o&j))for(k=g;k>=i;k--)q[j|o][max(k,l)]+=q[j][k];}A+=q[t][g];}
int main(){int i,N=0;scanf("%d%d",&n,&c);g=n*c;for(i=1;i<=n;i++)scanf("%d",&a[i]);sort(a+1,a+n+1);for(i=1;i<=n-1;i++)p[i]=i;do{memset(q,0,sizeof(q));q[0][a[n]*n]=1;S();N++;}while(next_permutation(p+1,p+n));double R=1.0/N;for(i=1;i<=n-1;i++)R*=(1.0/c);printf ("%.12lf\n",R*A);}