program p1096;
var n,m,x,s:integer;
    a:array[1..1000,1..2] of integer;
    b:array[1..1000,1..4] of integer;
procedure init;
var i:integer;
begin
     readln(n);
     for i:=1 to n do read(a[i,1],a[i,2]);
     read(m,b[1,1],b[1,2]);
     if (m=b[1,1])or(m=b[1,2]) then
     begin
          writeln(0);
          halt;
     end;
end;
procedure print(p:integer);
var i:integer;
begin
     for i:=1 to x do if i=b[p,4] then
     begin
          inc(s);
          print(i);
     end;
     if b[p,4]=-1 then writeln(s);
     writeln(b[p,3]);
end;
procedure solve;
var i,j:integer;
    out:boolean;
begin
     out:=false;i:=1;x:=1;
     while (not(out))and(i<=x) do
     begin
          for j:=1 to n do if j<>b[i,3] then
              if (b[i,1]=a[j,1])or
                 (b[i,1]=a[j,2])or
                 (b[i,2]=a[j,1])or
                 (b[i,2]=a[j,2]) then
                 begin
                      inc(x);
                      b[x,1]:=a[j,1];
                      b[x,2]:=a[j,2];
                      b[x,3]:=j;
                      if b[i,4]=0 then b[x,4]:=-1
                                  else b[x,4]:=i;
                      if (b[x,1]=m)or(b[x,2]=m) then
                      begin
                           out:=true;
                           break;
                      end;
                 end;
          inc(i);
     end;
     s:=1;
     if out then print(x)
            else writeln('IMPOSSIBLE');
end;
begin
     init;
     solve;
end.