#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,t ,sum;
    scanf("%d%d%d",&a,&b,&t);
    sum=a*t*0.5+b*t*0.5;
    printf("%d",sum);
    return 0;
}