program solitario;
var N,M,a:longint;
    
begin
    (*assign(input,  'input.txt');  reset(input);
    assign(output, 'output.txt'); rewrite(output);*)
	readln(N,M);
	 if M<N then begin a:=N; N:=M; M:=a; end;
	 if (N<=2) or ((N=3) and (M=3)) then writeln(N* (M-(M div 3)))
	                            else
                                  if N*M >25 then writeln(N*M*11 div 20+1)
                                             else if N=3 then writeln (N*M*2 div 3 - M div 3 )
                                                         else if (N=4) and( M mod 3 =0 ) then writeln((N-1)* (M-(M div 3))+1)
                                                                                         else writeln((N-1)* (M-(M div 3)));
                                                         
end.