#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 */   
    int a,b,t,n,r;
    cin>>a;
    cin>>b;
    cin>>t;
    n=a*t + b* t;
    n=n/2;
    r=(pow(10,9)+7);
    n=n % r;
    cout<<n;
    return 0;
}