function processData(input) {
    var [a, b, t] = input.split(' ').map(Number);
    var p = ((0.5 * a * t) + (0.5 * b * t)) % (Math.pow(10, 9) + 7);
    console.log(p);
} 

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

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