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 count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }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 count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }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 count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }else{
      System.out.println(-1);
    }
  } main {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();
    int count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }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 count = 0; count 0 while(sc.hasNext()){
      count += sc.nextInt();
    } sc.hasNext() sc.hasNext sc {
      count += sc.nextInt();
    } count += sc.nextInt() count sc.nextInt() sc.nextInt sc sc.close() sc.close sc if(count <= n){
      System.out.println(n - count);
    }else{
      System.out.println(-1);
    } count <= n count n {
      System.out.println(n - count);
    } System.out.println(n - count) System.out.println System.out System System.out n - count n count {
      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 sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();
    int count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }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 count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }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 count = 0;
    while(sc.hasNext()){
      count += sc.nextInt();
    }
    sc.close();
    
    if(count <= n){
      System.out.println(n - count);
    }else{
      System.out.println(-1);
    }
  }
}