import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int M = sc.nextInt(); M sc.nextInt() sc.nextInt sc int sum = 0; sum 0 for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        } int i = 0; int i = 0; i 0 i < M i M i++ i++ i {
            sum += sc.nextInt();
        } {
            sum += sc.nextInt();
        } sum += sc.nextInt() sum sc.nextInt() sc.nextInt sc if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        } N < sum N sum {
            System.out.println(-1);
        } System.out.println(-1) System.out.println System.out System System.out -1 1 if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        } N > sum N sum {
            System.out.println(N - sum);
        } System.out.println(N - sum) System.out.println System.out System System.out N - sum N sum {
            System.out.println(0);
        } System.out.println(0) System.out.println System.out System System.out 0 String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;

        for (int i = 0; i < M; i++) {
            sum += sc.nextInt();
        }

        if (N < sum) {
            System.out.println(-1);
        } else if (N > sum) {
            System.out.println(N - sum);
        } else {
            System.out.println(0);
        }
    }
}