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


int main() {
    float a, b;
    unsigned long long t;
    int vcount;
    
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    scanf ("%f %f %lld\n", &a,&b, &t);
    vcount = (0.5 * a * t) + (0.5 * b * t);
    printf("%d\n", vcount % (1000000007));
    
    return 0;
}