#include <iostream>
#include <cmath>

using namespace std;

int main(){
    long long factor = pow(10,9) + 7;
    double a, b, t;
    
    cin >> a >> b >> t;
    
    int expect = 0.5 * (a + b) * t;
    
    double cells = expect % factor;
    
    cout << cells << endl;
    return 0;
}