REST to create folder in doc libraries

Hope you have idea how to create SharePoint App.
If you are using NAPA tool then replace below codes in APP.js file.
So i am just focusing on the REST script part to do the activities

I recommend to go through REST API for SP document libraries before proceed.

HTML file in Default.aspx 
<div>
    <p>
        <b>Create Folder</b>
        <br />
        <input type="text" value="List Name Here" id="CreateFolder" />
        <button id="btnclick">Create Folder</button>
    </p>
</div>
----------------------
JS script in app.js (Paste the below methods in app.js file) Refer here
-----------------------
function FolderCreation()
{
    var executor;
    var getfoldername= document.getElementById("CreateFolder").value;
    // Initialize the RequestExecutor with the app web URL.
    executor = new SP.RequestExecutor(appweburl);
    executor.executeAsync({
  url: appweburl + "/_api/SP.AppContextSite(@target)/web/GetFolderByServerRelativeUrl('lib')/folders?@target='" + hostweburl + "'",
        method: "POST",
        body: "{ '__metadata':{ 'type': 'SP.Folder' }, 'ServerRelativeUrl':'Folder B' }",
}
          headers:  
           {
             "accept": "application/json; odata=verbose",
    "content-type": "application/json; odata=verbose"
            },  
success: FoldersSuccessHandler,
        error: FoldersErrorHandler
    });
}  
//Populate the selectFolders control after retrieving all of the folders.
function FoldersSuccessHandler(data) {
    alert("Folder Created successfully in Library");
}
function FoldersErrorHandler(data, errorCode, errorMessage) {
    alert("Could not Create a Folder in  Library: " + errorMessage);
}