
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStreamReader;


class Main{
    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int N = in.nextInt();
        int M = in.nextInt();
        int[] arr = new int[M];
        int sum = 0;
        for (int i = 0; i < M; i++) {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        }

        System.out.println((sum <= N )?((N-sum)):(-1));


    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
} import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.StringTokenizer; StringTokenizer java.util import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io class Main{
    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int N = in.nextInt();
        int M = in.nextInt();
        int[] arr = new int[M];
        int sum = 0;
        for (int i = 0; i < M; i++) {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        }

        System.out.println((sum <= N )?((N-sum)):(-1));


    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
} Main public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int N = in.nextInt();
        int M = in.nextInt();
        int[] arr = new int[M];
        int sum = 0;
        for (int i = 0; i < M; i++) {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        }

        System.out.println((sum <= N )?((N-sum)):(-1));


    } main {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int N = in.nextInt();
        int M = in.nextInt();
        int[] arr = new int[M];
        int sum = 0;
        for (int i = 0; i < M; i++) {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        }

        System.out.println((sum <= N )?((N-sum)):(-1));


    } InputReader in = new InputReader(System.in); in new InputReader(System.in) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) int N = in.nextInt(); N in.nextInt() in.nextInt in int M = in.nextInt(); M in.nextInt() in.nextInt in int[] arr = new int[M]; arr new int[M] M int sum = 0; sum 0 for (int i = 0; i < M; i++) {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        } int i = 0; int i = 0; i 0 i < M i M i++ i++ i {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        } {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        } sum = sum + in.nextInt() sum sum + in.nextInt() sum in.nextInt() in.nextInt in System.out.println((sum <= N )?((N-sum)):(-1)) System.out.println System.out System System.out (sum <= N )?((N-sum)):(-1) (sum <= N ) sum N ((N-sum)) N sum (-1) 1 String[] args args static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    } InputReader public BufferedReader reader; reader public StringTokenizer tokenizer; tokenizer public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } InputReader {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } reader = new BufferedReader(new InputStreamReader(stream), 32768) reader new BufferedReader(new InputStreamReader(stream), 32768) tokenizer = null tokenizer null InputStream stream stream public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } next {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } tokenizer == null || !tokenizer.hasMoreTokens() tokenizer == null tokenizer null !tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } IOException e {
                    throw new RuntimeException(e);
                } throw new RuntimeException(e); new RuntimeException(e) {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer = new StringTokenizer(reader.readLine()) tokenizer new StringTokenizer(reader.readLine()) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer public int nextInt() {
            return Integer.parseInt(next());
        } nextInt {
            return Integer.parseInt(next());
        } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.io.InputStreamReader;


class Main{
    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int N = in.nextInt();
        int M = in.nextInt();
        int[] arr = new int[M];
        int sum = 0;
        for (int i = 0; i < M; i++) {
            //arr[i] = in.nextInt();
            sum = sum + in.nextInt();
        }

        System.out.println((sum <= N )?((N-sum)):(-1));


    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}
