function Main( input ){

    input = input.split( '\n' )

    var a = Number( input[0] )
    var b = Number( input[1] )
    var c = Number( input[2] )

    if( ( b - a ) === ( c - b ) ) console.log( 'YES' )
    else console.log( 'NO' )

}

Main( require( 'fs' ).readFileSync( '/dev/stdin', 'utf8' ) )