#include <iostream>
using namespace std;

int main() {
    string S;
    int N;
    cin >> S >> N;
   int a = 0, b = 0;
    int i = 0;
     while (i < S.size() && isdigit(S[i])) {
       
     a = a * 10 + (S[i] - '0');
        i++;
    }
    i++;

    while (i < S.size() && isdigit(S[i])) {
        b = b * 10 + (S[i] - '0');
        i++;
    }
      int d = b - a;
    int sum = (N * (2 * a + (N - 1) * d)) / 2;

    

cout << sum << endl;

    return 0;
}