#include<cstdio>
      #include<algorithm>
      using namespace std;
      main()
      {
          FILE *f=fopen("dezastru.in","r"), *g=fopen("dezastru.out","w");
          int n,i,a[300],k,j;
          float v[300],s[300],x;
          fscanf(f,"%d %d ",&n,&k);
          for(i=0;i<=n;i++)
              a[i]=i+1;
           for(i=0;i<n;i++)
              fscanf(f,"%f ",&v[i]);
          i=0;
          do
          {       s[i]=1;
                  j=0;
                  while(j<k){ s[i]*=v[a[j]-1]; j++; }
                  i++; 
//                fprintf(g,"%f ",s[i-1]);                
          } while ( next_permutation( a, a + n ) );
          j=i;
          x=0;
         for(i=0;i<j;i++)
          x+=s[i];
          fprintf(g,"%f",x/j);
          return 0;
}