import java.io.OutputStream;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.PrintWriter;
    import java.util.InputMismatchException;
    import java.io.IOException;
    import java.io.InputStream;
    import java.util.*;
    /**
     * Built using CHelper plug-in
     * Actual solution is at the top
     *
     * @author Manan Patel
     */
    public class Main{
        public static void main(String[] args) {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            InputReader sc = new InputReader(inputStream);
            PrintWriter w = new PrintWriter(outputStream);
            solution solver = new solution();
            //int t = sc.nextInt();
            //for(int i = 1 ; i <= t ; i++)
                solver.solve(1 , sc , w);
            
            w.close();
        }

        static class solution {
            public void solve(int tc, InputReader sc, PrintWriter w) {
                int n = sc.nextInt() , m = sc.nextInt();
                long sum = 0;
                for(int i = 0 ; i < m ; i++)
                    sum += sc.nextInt();
                int d = (int) (n - sum);
                w.println(d >= 0 ? d : "-1");
            }
        }

        static class InputReader {
            private InputStream stream;
            private byte[] buf = new byte[1024];
            private int curChar;
            private int numChars;
            private InputReader.SpaceCharFilter filter;

            public InputReader(InputStream stream) {
                this.stream = stream;
            }

            public int read() {
                if (numChars == -1)
                    throw new InputMismatchException();

                if (curChar >= numChars) {
                    curChar = 0;
                    try {
                        numChars = stream.read(buf);
                    } catch (IOException e) {
                        throw new InputMismatchException();
                    }

                    if (numChars <= 0)
                        return -1;
                }
                return buf[curChar++];
            }

            public int nextInt() {
                int c = read();

                while (isSpaceChar(c))
                    c = read();

                int sgn = 1;

                if (c == '-') {
                    sgn = -1;
                    c = read();
                }

                int res = 0;
                do {
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    res *= 10;
                    res += c - '0';
                    c = read();
                }
                while (!isSpaceChar(c));

                return res * sgn;
            }

            public long nextLong() {
                int c = read();
                while (isSpaceChar(c))
                    c = read();
                int sgn = 1;
                if (c == '-') {
                    sgn = -1;
                    c = read();
                }
                long res = 0;

                do {
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    res *= 10;
                    res += c - '0';
                    c = read();
                }
                while (!isSpaceChar(c));
                return res * sgn;
            }

            public String readString() {
                int c = read();
                while (isSpaceChar(c))
                    c = read();
                StringBuilder res = new StringBuilder();
                do {
                    res.appendCodePoint(c);
                    c = read();
                }
                while (!isSpaceChar(c));

                return res.toString();
            }

            public boolean isSpaceChar(int c) {
                if (filter != null)
                    return filter.isSpaceChar(c);
                return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
            }

            public String next() {
                return readString();
            }

            public interface SpaceCharFilter {
                public boolean isSpaceChar(int ch);

            }

        }
    }



Measured memory_bytes: 33152