My android Studio shows this error.

> 
> All com.android.support libraries must use the exact same version
>  specification (mixing versions can lead to runtime crashes). Found
>  versions 27.1.1, 26.1.0. Examples include
>  com.android.support:animated-vector-drawable:27.1.1 and
>  com.android.support:support-media-compat:26.1.0 less... (Ctrl+F1) 
>  There are some combinations of libraries, or tools and libraries, that
>  are incompatible, or can lead to bugs. One such incompatibility is
>  compiling with a version of the Android support libraries that is not
>  the latest version (or in particular, a version lower than your
>  targetSdkVersion).
> 
> 
> 

```
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    implementation 'com.google.firebase:firebase-core:15.0.0'

```