Android : how to fill the value in build.gradle in Android studio

there are lines in build.gradle file

buildscript {
    repositories {
        mavenCentral()
    }

    dependencies {
        classpath 'com.android.tools.build:gradle:1.21'
    }

However, when building the project, the error notes say, "Error:Could not find com.android.tools.build:gradle:1.21."

So how to find the gradle number that installed in my Android Studio?

source code is from the official training: http://developer.android.com/downloads/samples/DisplayingBitmaps.zip