#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#define vivek 1000000007;
using namespace std;



int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int a,b,t;
    
    cin>>a>>b>>t;
    
    int i;
    float f=0;;
    for(i=1;i<=t;++i)
        {
           f += (0.5 *a*i)+ (0.5*b*i);
    }
    int ans = (int)f;
    
    int w= ans % vivek;  
    cout<<w;
    return 0;
}