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


/*
#include <inttypes.h>
uint64_t t;
printf("%" PRIu64 "\n", t);
*/

int main() {

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */    

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