#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#define M 1000000007
int main() {

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    int a,b;
    long long int t,result;
    scanf("%d %d %lld",&a,&b,&t);
    result=(t%M)*0.5*(t%M)*(a+b);
printf("%lld\n",result);


return 0;
}