function processData(input) {
    let data = input.split(" ");
    let factorA = 0.5;
	let factorB = 0.5;
	console.log(factorA*data[0]*data[2] + factorB*data[1]*data[2]);
} 

process.stdin.resume();
process.stdin.setEncoding("ascii");
_input = "";
process.stdin.on("data", function (input) {
    _input += input;
});

process.stdin.on("end", function () {
   processData(_input);
});