// editor5
import java.util.*;

class BudgetManager {
    private int initialBudget;
    private int remainingBalance;

    public BudgetManager(int initialBudget) {
        this.initialBudget = initialBudget;
        this.remainingBalance = initialBudget;
    }

    public boolean addExpense(int amount) {
        if (amount > initialBudget) {
            return false; // invalid expense
        }
        remainingBalance -= amount;
        return true;
    }

    public int getRemainingBalance() {
        return remainingBalance;
    }
}

public class Main {   // ✅ Fix: Capitalized class name
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int initialBudget = sc.nextInt();
        int numberOfExpenses = sc.nextInt();

        if (numberOfExpenses < 0) {
            System.out.println("Invalid input");
            return;
        }

        BudgetManager manager = new BudgetManager(initialBudget);

        for (int i = 0; i < numberOfExpenses; i++) {
            int expense = sc.nextInt();
            if (!manager.addExpense(expense)) {
                System.out.println(-1); // ✅ Fix: print -1 if expense > budget
                return; // stop further processing
            }
        }

        System.out.println(manager.getRemainingBalance());
    }
}