program mire;
var n,m,x,i,j,k,l,r:int64;
     ok:boolean;
function cmmdc(a,b:int64):int64;
begin
  if a<b then
    begin
      l:=a;
      a:=b;
      b:=l;
    end;
  repeat
    r:=a mod b;
    a:=b;
    b:=r;
  until b=0;
cmmdc:=a;
end;
begin
    readln(n);
    readln(m);
    ok:=false; i:=m; j:=n;
 while j<=m do
   begin
    while i>=n do
     begin
     if cmmdc(j,i)=1 then
      begin
        if (j=n)  and (i=m)  then
          write(i-j+1)
         else
           write(i-j);
         ok:=true;
         break;
      end;
       i:=i-1;
     end;
     j:=j+1;
      if ok then
        break;
   end;
 if not ok then
   write(-1);
end.