Program fail1;
label 1;    var i,n:integer;  m:array [1..1000] of integer;
procedure exchange(var a,b:integer);
var c:integer;
begin  c:=a; a:=b; b:=c; end;
begin
1: writeln ('Vvedite N, kratnoe 3: '); readln(n) ;
if n mod 3<>0 then begin
writeln ('Vi vveli cislo NE kratnoe 3. Povtorite popitku...'); goto 1;  end;
writeln('Elementi massiva:');
for i:=1 to n do m[i]:=i;
for i:=1 to n do  write(m[i],' ');
 
i:=1;
repeat
exchange(m[i],m[i+2]); i:=i+3;
until i>n;
writeln;
writeln('Ishodnii massiv:');
for i:=1 to n do write(m[i],' ');
readln;
end.