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


int main() {
    unsigned a,b;
    long int t, val;cin>>a>>b>>t;
    val=0.5*(a+b);
    val=val*t;
    val=val%1000000007;
    cout<<val;
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    return 0;
}