#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 f1,f2,t;
    cin>>f1>>f2>>t;
    
    int e;
    
    //e= int((0.5*t*(f1+f2))%(pow(10,9)+7));
    e=int(0.5*t*(f1+f2))%int(pow(10,9)+7);
    cout<<e<<endl;
    return 0;
}