#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main() {

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
   int a,b,t;
       double c;
    scanf("%d %d %d",&a,&b,&t);
    c=fmod(((0.5*a*t)+(0.5*b*t)),(pow(10,9)+7));
    printf("%d",(int)c);
    
    return 0;
}