// for AtCoder
"use strict";(function(){let n,t,i;this.input?n=this.input:"function"==typeof require&&(n=require("fs").readFileSync("/dev/stdin","utf8"));let o=0;this.debug=function t(i){"object"==typeof i?t(i[0]):n=i.trim()},this.next=s,this.nextNumber=function(){return+s()},this.write=function(n){void 0===n&&(n="");var t=(r+=n).match(/([\s\S]*)\n([^\n]*)/);t&&(log(t[1]),r=t[2])},this.flush=function(n){void 0===n&&(n="");""!=(r+=n)&&(log(r),r="")},this.getLine=function(){t||(t=n.split(/[\n\r]+/));return i=null,t[e++]},this.log=console.log.bind(console),this.err=console.error.bind(console);let e=0,r="";function s(){return t||(t=n.split(/[\n\r]+/)),(!i||i.length-1<=o)&&(o=0,i=t[e++].split(/\s+/)),i[o++]}function u(n,t,i){for(var o=n;o<t&&i(o)!==BREAK;o++);}function l(n,t,i){for(var o=t-1;o>=0&&i(o)!==BREAK;o--);}this.FOR=u,this.RFOR=l,this.REP=function(n,t){u(0,n,t)},this.RREP=function(n,t){l(0,n,t)},this.CONTINUE=0,this.BREAK=1,this.range=function n(t,i){if(void 0===i)return n(0,t);return Array.apply(null,{length:i-t}).map((n,t)=>t)},"function"==typeof main&&main()}).call("undefined"!=typeof window?window:global);
//

let a=nextNumber()
let b=nextNumber()
let c=nextNumber()
log(Math.floor((a+b)*c/2))
