'use strict'

function main(s) {

  s = s.split('\n').map(x => Number(x));
  const a = s[0];
  const b = s[1];
  const h = s[2];

  const area = ((a + b) * h) / 2
  console.log(area);
}

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