process.stdin.resume();
process.stdin.setEncoding('ascii');

var input_stdin = "";
var input_stdin_array = "";
var input_currentline = 0;

process.stdin.on('data', function (data) {
    input_stdin += data;
});

process.on('SIGINT', function(){
    input_stdin_array = input_stdin.split("\n");
    main();
    process.exit();
});

process.stdin.on('end', function () {
    input_stdin_array = input_stdin.split("\n");
    main();    
});

function readLine() {
    return input_stdin_array[input_currentline++];
}

/////////////// ignore above this line ////////////////////


function main() {
	var b1,b2,h;
	b1 = parseInt(readLine());
	b2 = parseInt(readLine());
	h = parseInt(readLine())/2;
    var ans = (b1+b2)*h;
	
	console.log(ans.toString()); 
    process.exit();
}