program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;

const
  INF = MAXINT - 100;
  MAXIN = 10000;
  eps = 1e-7;

var
  n, m, e, i, j, u, a1, b1, p, s, ind: integer;
  a: array [1..MAXIN, 1..MAXIN] of integer;
  d, d1, ans, mark: array [1..MAXIN] of integer;
  min: extended;

begin
  readln(n, m, e);

  for i := 1 to MAXIN do
    for j := 1 to MAXIN do
      a[i][j] := 0;

  for i := 1 to m do
  begin
    readln(a1, b1, d[i]);
    d1[i] := d[i];
    a[a1][b1] := 1;
    a[b1][a1] := 1;
    mark[i] := 0;
  end;

  for i := 1 to e do
    read(ans[i]);

  readln(p, s);
  d[p] := 0;
  d1[s] := 0;

  while (true) do
  begin
    ind := -1;
    min := INF;
    for i := 1 to m do
      if (mark[i] = 0) and (d[i] < min) then
      begin
        ind := i;
        min := d[i];
      end;
    if (ind = -1) then
      break;
    mark[ind] := 1;
    for u := 1 to m do
      if (a[ind][u] <> -1) and (mark[u] = 0) and (d[u] > d[ind] + a[ind][u]) then
        d[u] := d[ind] + a[ind][u];
  end;

  while (true) do
  begin
    ind := -1;
    min := INF;
    for i := 1 to m do
      if (mark[i] = 0) and (d1[i] < min) then
      begin
        ind := i;
        min := d1[i];
      end;
    if (ind = -1) then
      break;
    mark[ind] := 1;
    for u := 1 to m do
      if (a[ind][u] <> -1) and (mark[u] = 0) and (d1[u] > d1[ind] + a[ind][u]) then
        d1[u] := d1[ind] + a[ind][u];
  end;

  min := MAXINT;

  for i := 1 to e do
    if (d[ans[i]] < d1[ans[i]]) then
      if ((d[ans[i]] * 160000) / d1[ans[i]] - min < eps) then
        min := (d[ans[i]] * 160000) / d1[ans[i]] / 1000;

  if (min = MAXINT) then
    writeln('IMPOSSIBLE')
  else
    writeln(min:0:7);

  readln;
  readln;
end.