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

int main() {
    double t,sum=0,l;
    int k;
    
    
    int a,b;
    scanf("%d%d%lf",&a,&b,&t);
    sum=((0.5*a*t)+(0.5*b*t));
    l=pow(10,9)+7;
    k=fmod(sum,l);
    printf("%d",k);

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    
    return 0;
}