/**
 * Implements hook_views_pre_execute().
 */
function MODULENAME_views_pre_execute(&$view) {
  // Set the first contextual filter value.
  $view->args[0] = 1;
}

/**
 * Implements hook_views_pre_execute().
 */
function mymodule_views_pre_view(&$view, &$display_id, &$args) {
  if ($view->name == 'VIEWNAME' && $view->current_display == 'DISPLAYNAME') {
    $url = arg();
    if (isset($url[1])) {
      // YOU MIGHT HAVE TO ADJUST $url[1] TO MATCH YOUR URL ALIAS.
      if ($normal_path = drupal_get_normal_path($url[1])) {
        $path_args = explode('/', $normal_path);
      }
    }
    if (isset($path_args[1])) {
      $view->args[0] = $path_args[1];
    }
  }
}