#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <sstream>
#include <string>
using namespace std;


int main() {
    string values;
    int a, b;
    long long t;
    
    getline(cin, values);
    istringstream ss(values);
    ss >> a;
    ss >> b;
    ss >> t;
    
    int expected = ((a + b) * (.5 * t));
    int e = expected % (10*10*10*10*10*10*10*10*10 +7);
    cout << e;
    
    return 0;
}