import 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[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

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

        int n = sc.nextInt();
        int m = sc.nextInt();

        int[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

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

        int n = sc.nextInt();
        int m = sc.nextInt();

        int[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

        if(ans < 0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        }
    } main {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        int m = sc.nextInt();

        int[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

        if(ans < 0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        }
    } 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[] a = new int[m]; a new int[m] m for(int i = 0; i < m; i++)
            a[i] = sc.nextInt(); int i = 0; int i = 0; i 0 i < m i m i++ i++ i a[i] = sc.nextInt(); a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc int ans = n; ans n for(int i = 0; i < m; i++)
            ans = ans - a[i]; int i = 0; int i = 0; i 0 i < m i m i++ i++ i ans = ans - a[i]; ans = ans - a[i] ans ans - a[i] ans a[i] a i if(ans < 0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        } ans < 0 ans 0 {
            System.out.println(-1);
        } System.out.println(-1) System.out.println System.out System System.out -1 1 {
            System.out.println(ans);
        } System.out.println(ans) System.out.println System.out System System.out ans 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[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

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

        int n = sc.nextInt();
        int m = sc.nextInt();

        int[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

        if(ans < 0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        }
    }
} Main
import 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[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

        if(ans < 0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        }
    }
}