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


int main() 
{
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */ 
    unsigned long long a, b, t, growthA, growthB, numOfCells;
    
    cin >> a; cin >> b; cin >> t;
    
    growthA = a * t;
    growthB = b * t;
    numOfCells = (growthA + growthB) / 2;
    cout << numOfCells % (long long)(pow(10,9) + 7);
        
    return 0;
}