package grigorieva.olga;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class OAiPVariant7Program4 {
    public static final int MaxSize = 20;

    public static void main(String[] args) throws IOException {
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Данная программа вычисляет: -A1/1! + A2/2! -...+ (-1)^N*AN/N!");
        System.out.println("Введите N: ");
        boolean isCorrect = true;
        String inputn = bufferedReader.readLine();
        int n = Integer.parseInt(inputn);
        do {
            System.out.println("Введите N: ");
            try {
                n = Integer.parseInt(inputn);
                if (n > 0 && n < MaxSize) {
                    isCorrect = false;
                }
            } catch (Exception e) {
            }
        } while (isCorrect);
        int[] aArray = new int[n];

        for (int i = 0; i < n; i++) {
            System.out.println("Введите A" + (i + 1) + ": ");
            String ai = bufferedReader.readLine();
            aArray[i] = Integer.parseInt(ai);
        }
        double sum = 0;
        for (int i = 0; i < n; i++) {

            double step = 1;
            for (int j = 0; j < i; j++) {
                step = -step ;
            }

            double facto = 1;
            for (int j = 2; j < i + 1; j++) {
                facto = facto * j;
            }

            double znach = step * aArray[i] / facto;
            sum = sum + znach;
        }
        System.out.println("Сумма = " + sum);
    }
}