{dezastru preONI2007}
{$N+}

var a:array[1..25] of real;
    marc:array[0..25,0..25] of extended;
    suma:extended;
    nr,n,k:byte;
    f,g:text;

procedure citire;
var i:byte;
begin
assign(f,'dezastru.in'); reset(f);
readln(f,n,k);
for i:=1 to n do read(f,a[i]);
close(f);
end;

procedure formare;
var i,j:byte;
begin
for i:=0 to n do marc[i,0]:=1;
for i:=1 to k-1 do
    for j:=i+1 to k do
        marc[i,j]:=marc[i-1,j]+marc[i-1,j-1]*a[i];
for i:=k+1 to n do
    for j:=1 to k do
        marc[i,j]:=marc[i-1,j]+marc[i-1,j-1]*a[i];
end;

function combinari:extended;
var prod:extended;
    i:byte;
begin
for i:=1 to k do
    prod:=prod*((n-i+1)/i);
combinari:=prod;
end;


procedure afisare;
var val,valoare:extended;
begin
assign(g,'dezastru.out'); rewrite(g);
valoare:=combinari;
val:=marc[n,k]/valoare;
writeln(g,val:2:6);
close(g);
end;

begin
suma:=0;
nr:=0;
citire;
formare;
end.