{$N+}
const nmax=26;
var f:text;
    comb:array[0..nmax,0..nmax] of longint;
    i,n,k,j:byte;
    v:array[1..nmax] of real;
    mat: array[0..nmax,0..nmax] of extended;
Begin
     assign(f,'dezastru.in');reset(f);
     readln(f,n,k);
     for i:=1 to n do
         read(f,v[i]);
     close(f);
     comb[1,0]:=1;  comb[1,1]:=1;
     mat[1,1]:=v[1]; mat[1,0]:=1;
     for i:=2 to n do
     begin
          comb[i,0]:=1;
          mat[i,0]:=1;
          j:=1;
          while (j<=i) and (j<=k) do begin
                mat[i,j]:=mat[i-1,j]+mat[i-1,j-1]*v[i];
                comb[i,j]:=comb[i-1,j]+comb[i-1,j-1];
                j:=j+1;
          end;
     end;
     assign(f,'dezastru.out');rewrite(f);
     write(f,mat[n,k]/comb[n,k]:0:6);
     close(f);
End.