"use strict";

function main(arg){
  const input = arg.trim().split('\n').map(_=>parseInt(_, 10));
  const a = input[0];
  const b = input[1];
  const h = input[2];
  
  console.log((a+b)*h / 2)
}

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