import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    }

} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.ArrayList; ArrayList java.util import java.util.List; List java.util public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    }

} Main public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    } main {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    } InputStreamReader is = new InputStreamReader(System.in); is new InputStreamReader(System.in) BufferedReader reader = new BufferedReader(is); reader new BufferedReader(is) List<String> mylist = new ArrayList<>(); mylist new ArrayList<>() String strinput = reader.readLine(); strinput reader.readLine() reader.readLine reader while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        } strinput != null strinput null {

            mylist.add(strinput);
            strinput = reader.readLine();

        } mylist.add(strinput) mylist.add mylist strinput strinput = reader.readLine() strinput reader.readLine() reader.readLine reader String arr01[] = mylist.get(0).split(" "); arr01 mylist.get(0).split(" ") mylist.get(0).split mylist.get(0) mylist.get mylist 0 " " String arr02[] = mylist.get(1).split(" "); arr02 mylist.get(1).split(" ") mylist.get(1).split mylist.get(1) mylist.get mylist 1 " " int n = Integer.valueOf(arr01[0]); n Integer.valueOf(arr01[0]) Integer.valueOf Integer arr01[0] arr01 0 int m = Integer.valueOf(arr01[1]); m Integer.valueOf(arr01[1]) Integer.valueOf Integer arr01[1] arr01 1 int mysum = 0; mysum 0 for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        } int i = 0; int i = 0; i 0 i < m i m i++ i++ i {
            mysum += Integer.valueOf(arr02[i]);

        } {
            mysum += Integer.valueOf(arr02[i]);

        } mysum += Integer.valueOf(arr02[i]) mysum Integer.valueOf(arr02[i]) Integer.valueOf Integer arr02[i] arr02 i if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        } mysum > n mysum n {
            System.out.println(-1);
        } System.out.println(-1) System.out.println System.out System System.out -1 1 {
            System.out.println(n - mysum);
        } System.out.println(n - mysum) System.out.println System.out System System.out n - mysum n mysum String[] args args public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    }

} public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    }

} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");
        String arr02[] = mylist.get(1).split(" ");
        int n = Integer.valueOf(arr01[0]);
        int m = Integer.valueOf(arr01[1]);

        int mysum = 0;
        for (int i = 0; i < m; i++) {
            mysum += Integer.valueOf(arr02[i]);

        }

        if (mysum > n) {
            System.out.println(-1);
        } else {
            System.out.println(n - mysum);
        }
    }

}
