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