function Main(input){
    input=input.split("\n");
    tmp=input[0].split(" ");
    var a = parseInt(tmp[0], 10);
    var b = parseInt(tmp[1], 10);
    var c = parseInt(tmp[2], 10);
    if(b-a == c-b){
	console.log("YES");
    }else{
	console.log("NO");
    }
}

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