Retrive a Folder in Document Library using REST

//Retrieve all of the folders from root Site

function getFolders() {

var executor;

// Initialize the RequestExecutor with the app web URL.

executor = new SP.RequestExecutor(appweburl);

executor.executeAsync({

url: appweburl + "/_api/SP.AppContextSite(@target)/web/getfolderbyserverrelativeurl('/GowthamLibrary')/folders?@target='" + hostweburl + "'",

method: "GET",

headers: {

"Accept": "application/json; odata=verbose"

}, success: FoldersSuccessHandler,

error: FoldersErrorHandler

});

}

im creating app to retrive the folders from Library but im getting error.i think uRL formation is wrong.anyone can help me to fix this?