function Main(input) {
    var array = input.replace(/\r?\n+$/, '').split(/\r?\n/).map(Number);
    console.log((array[0] + array[1]) * array[2] / 2);
}

var character = "";
process.stdin.on('data', function(char){ character += char });
process.stdin.on('end', function(){ Main(character); });
process.on('SIGINT', function() { Main(character); process.exit(0); });