'use strict'

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

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