Invalid variable name error in Android build.grade

My code:

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.2.0'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    } }

allprojects {
    repositories {
        jcenter()
    } }

task clean(type: Delete) {
    delete rootProject.buildDir }

Here the error log:

Error:(1, 0) Cause: startup failed: build file 'D:\Doctor\build.gradle': 1: Invalid variable name. Must start with a letter but was: . At [1:1] @ line 1, column 1. // Top-level build file where you can add configuration options common to all sub-projects/modules. ^

1 error

<a href="openFile:D:\Doctor\build.gradle">Open File</a>

Copyright Notice：Content Author:「ZhangKuixun」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/40711981/invalid-variable-name-error-in-android-build-grade