import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    }

    static class BHomework {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    }

    static class BHomework {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out Scanner in = new Scanner(inputStream); in new Scanner(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) BHomework solver = new BHomework(); solver new BHomework() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class BHomework {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        }

    } BHomework public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        } solve {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        } int n = in.nextInt(); n in.nextInt() in.nextInt in int m = in.nextInt(); m in.nextInt() in.nextInt in int acc = 0; acc 0 for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            } int i = 0; int i = 0; i 0 i < m i m i++ i++ i {
                acc += in.nextInt();
            } {
                acc += in.nextInt();
            } acc += in.nextInt() acc in.nextInt() in.nextInt in out.println(n < acc ? -1 : n - acc) out.println out n < acc ? -1 : n - acc n < acc n acc -1 1 n - acc n acc int testNumber testNumber Scanner in in PrintWriter out out public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    }

    static class BHomework {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    }

    static class BHomework {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author silviase
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        BHomework solver = new BHomework();
        solver.solve(1, in, out);
        out.close();
    }

    static class BHomework {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            int acc = 0;
            for (int i = 0; i < m; i++) {
                acc += in.nextInt();
            }
            out.println(n < acc ? -1 : n - acc);
        }

    }
}

