import java.util.*;


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


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