Flask - 404 not found

I am a newbie in the flask development this is my first program in the flask but it shows me this error:

The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.

& this is my code

from flask import Flask

app = Flask(__name__)


@app.route('/index')
def index():
    return 'Hello World'


if __name__ == '__main__':
    app.run(debug=True)