#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;
    long int t,sum;
    scanf("%d%d%ld",&a,&b,&t);
    //printf("%d %d %ld",a,b,t);
    sum = ((0.5)*a*t)+((0.5)*b*t);
       // finalsum = sum%(10^9+7);
        printf("%ld",sum%(10^9+7));
}