#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 */ 
    long long int x,y,t,a;
    cin>>x>>y>>t;
    a=((x+y)*0.5*t);
       a=fmod(a,1000000007);
    
    cout<<a;
    return 0;
}