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