var t,c,i,n,k:integer;
begin
read (n,k);
i:=1;

while i<n do
begin
If c<k then c:=c+1;
i:=i+c;
t:=t+1;
end;
If n=-1 then t:=-1;
writeln (t);

end.

Edited by author 12.09.2007 19:47