1.2.19

#include <iostream>
using namespace std;

int main() {
    // No II.19
    double a1, d, n, S;

    cout << "Enter a1, d, n = ";
    cin >> a1 >> d >> n;
    cout << endl;

    S = n/2 * (2 * a1 + (n - 1) * d);

    cout << "Result = "<< S << endl;
    return 0;
}