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Cod sursa(job #640436)
Program dezastru;
var a:array [0..25] of byte;
b:array [0..25] of double;
sum:double;
nr:longint;
n,k,i:byte;
fi,fo:text;
procedure back(l:byte);
var i:byte;
s:double;
begin
if l=k then begin
s:=1;
for i:=1 to k do s:=s*b[a[i]];
sum:=sum+s; inc(nr);
end
else for i:=a[l]+1 to n do begin
a[l+1]:=i;
back(l+1);
end;
end;
begin
assign(fi,'dezastru.in');
assign(fo,'dezastru.out');
reset(fi); rewrite(fo);
readln(fi,n,k);
for i:=1 to n do read(fi,b[i]);
back(0);
write(fo,sum/nr:0:6);
close(fo);
end.