import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    }
} import java.util.Scanner; Scanner 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 total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    } 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 total = 0; total 0 for(int i =0; i<m; i++) {
            total += sc.nextInt();
        } int i =0; int i =0; i 0 i<m i m i++ i++ i {
            total += sc.nextInt();
        } {
            total += sc.nextInt();
        } total += sc.nextInt() total sc.nextInt() sc.nextInt sc n -= total n total if(n >= 0) System.out.println(n);
        else System.out.println(-1); n >= 0 n 0 System.out.println(n) System.out.println System.out System System.out n System.out.println(-1) System.out.println System.out System System.out -1 1 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 total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int total = 0;
        for(int i =0; i<m; i++) {
            total += sc.nextInt();
        }
        n -= total;
        if(n >= 0) System.out.println(n);
        else System.out.println(-1);
    }
}