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


int main() {
    int a,b;
    unsigned long long t,temp;
    cin>>a>>b>>t;
    unsigned long long var,c;
    var=(a*t+b*t)/2;
    temp=pow(10,9);
    c=var%(temp+7);
    cout<<c;
    return 0;
}