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public static void main(String[] args) {
    int a;
    Scanner scanner = new Scanner(System.in);
    System.out.println("Enter number");
    a = scanner.nextInt();
    System.out.println(a * a);
}

int a;
int a1 = 2;
Scanner scanner = new Scanner(System.in);
System.out.println("Введите число");
a = scanner.nextInt();
System.out.println(Math.pow(a, a1));

int a;
Scanner scanner = new Scanner(System.in);
System.out.println("Enter number");
a = scanner.nextInt();
a = a * a;
System.out.println(a);