'use strict';

function Main(input) {
    const tmp = input.split('\n');
    const a = parseInt(tmp[0], 10);
    const b = parseInt(tmp[1], 10);
    const h = parseInt(tmp[2], 10);
    console.log((a + b) * h / 2);
}

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