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

int main() {
    char expression[50];
    scanf("%s",expression);
    
    int i=0, first_plus=0,a=0,b=0,n;
    
    while (expression[i] != '+') {
        a = a * 10 + (expression[i] - '0');
        i++;
    }

    i++;
    
    while (expression[i] != '+') {
        b = b * 10 + (expression[i] - '0');
        i++;
    }
    
    scanf("%d",&n);
    int sum = ((2*a + (n-1)*(b-a)) * n) / 2;
    printf("%d",sum);
  
    return 0;
}