function area(input) {
	var a = Number(input.split("\n")[0]);
	var b = Number(input.split("\n")[1]);
	var h = Number(input.split("\n")[2]);
	var S = (a+b)*h/2.0;
	console.log(S);
	return S;
}

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