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


int main() {
    int a,b,t;
    double res;
    cin>>a>>b>>t;
    res=(0.5*t)*(a+b);
    cout<<(int)res %(1000000007);
    return 0;
}