import java.util.*;
import java.lang.*;

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;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
if(sum>n) System.out.println(-1);
else {
System.out.println(n-sum);
}
  }

} import java.util.*; util.* java import java.lang.*; lang.* 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;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
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 = sc.nextInt();
    int m = sc.nextInt();
int sum=0;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
if(sum>n) System.out.println(-1);
else {
System.out.println(n-sum);
}
  } main {
  
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();
int sum=0;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
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 = sc.nextInt(); n sc.nextInt() sc.nextInt sc int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc int sum=0; sum 0 while(m--!=0){
int a = sc.nextInt();
sum+=a;
} m--!=0 m-- m 0 {
int a = sc.nextInt();
sum+=a;
} int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc sum+=a sum a if(sum>n) System.out.println(-1);
else {
System.out.println(n-sum);
} sum>n sum n 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 = sc.nextInt();
    int m = sc.nextInt();
int sum=0;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
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 = sc.nextInt();
    int m = sc.nextInt();
int sum=0;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
if(sum>n) System.out.println(-1);
else {
System.out.println(n-sum);
}
  }

} Main
import java.util.*;
import java.lang.*;

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;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
if(sum>n) System.out.println(-1);
else {
System.out.println(n-sum);
}
  }

}