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

int main() {
    int a,b,t;
    int c;
    scanf("%d\t,%d\t,%d\t",&a,&b,&t);
    c=((a*0.5*t)+(b*0.5*t))%(pow(10,9)+7);
    printf("%d",c);

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