program ZAD1_1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var m,n :integer;
S :REAL;
begin
  writeln('vvedit znachena n,m ');
  readln(n,m);
  S:=(M-(2*N-5))/2;
  WRITELN('S= ',S:2:5);
  READLN;
  end.