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();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    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();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    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();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    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 for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    } int i = 0; int i = 0; i 0 i < m i m i++ i++ i {
      n -= sc.nextInt();
    } {
      n -= sc.nextInt();
    } n -= sc.nextInt() n sc.nextInt() sc.nextInt sc int ans = n; ans n if(n < 0) ans = -1; n < 0 n 0 ans = -1 ans -1 1 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();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    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();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    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();
    for(int i = 0; i < m; i++) {
      n -= sc.nextInt();
    }
    int ans = n;
    if(n < 0) ans = -1;
    System.out.println(ans);
  }
}