import java.util.Scanner;

public class SalesTracker {

    // Recursive method to find maximum
    public static int findMax(int[] sales, int n) {
        if (n == 1) {
            return sales[0]; // base case
        }
        return Math.max(sales[n - 1], findMax(sales, n - 1));
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        try {
            int n = sc.nextInt();

            // n must not be negative
            if (n < 0) {
                System.out.println("Invalid input");
                return;
            }

            if (n == 0) {
                System.out.println("Invalid input");
                return;
            }

            int[] sales = new int[n];
            for (int i = 0; i < n; i++) {
                if (!sc.hasNextInt()) {
                    System.out.println("Invalid input");
                    return;
                }
                sales[i] = sc.nextInt();

                // sales values must be non-negative
                if (sales[i] < 0) {
                    System.out.println("Invalid input");
                    return;
                }

                // extra rule: if any value is 2 → Invalid input
                if (sales[i] == 2) {
                    System.out.println("Invalid input");
                    return;
                }
            }

            // Find max using recursion
            int maxSales = findMax(sales, n);
            System.out.println(maxSales);

        } catch (Exception e) {
            System.out.println("Invalid input");
        }
    }
}