process.stdin.resume();
process.stdin.setEncoding('utf8');
// Here your code !
process.stdin.on('data', function (chunk) {
    var lines = chunk.toString().split('\n');
    var a =parseInt(lines[0]);
    var b =parseInt(lines[1]);
    var h =parseInt(lines[2]);
    
    var ans = (a+b)*h/2;
    console.log(ans);
    
});
