function main(a){
	a = a.split("\n");
	a=stn(a);
	op(((a[0]+a[1])*a[2])/2);
}
function op(a){
	console.log(a);
}
function stn(a){
		return a.map(function(x){return parseInt(x,10);});
}
main(require("fs").readFileSync("/dev/stdin", "utf8"));