import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  }
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.io.IOException; IOException java.io public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  }
} Main public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  } main {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  } BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); in new BufferedReader(new InputStreamReader(System.in)) String[] str = in.readLine().split(" "); str in.readLine().split(" ") in.readLine().split in.readLine() in.readLine in " " int n = Integer.parseInt(str[0]); n Integer.parseInt(str[0]) Integer.parseInt Integer str[0] str 0 int m = Integer.parseInt(str[1]); m Integer.parseInt(str[1]) Integer.parseInt Integer str[1] str 1 String[] hw_days = in.readLine().split(" "); hw_days in.readLine().split(" ") in.readLine().split in.readLine() in.readLine in " " int sum = 0; sum 0 for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    } int i=0; int i=0; i 0 i<m i m i++ i++ i {
      sum += Integer.parseInt(hw_days[i]);
    } {
      sum += Integer.parseInt(hw_days[i]);
    } sum += Integer.parseInt(hw_days[i]) sum Integer.parseInt(hw_days[i]) Integer.parseInt Integer hw_days[i] hw_days i System.out.println(n-sum >= 0?n-sum:-1) System.out.println System.out System System.out n-sum >= 0?n-sum:-1 n-sum >= 0 n-sum n sum 0 n-sum n sum -1 1 String[] args args public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  }
} public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  }
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class Main {
  public static void main(String[] args) throws IOException {
    BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    String[] str = in.readLine().split(" ");
    int n = Integer.parseInt(str[0]);
    int m = Integer.parseInt(str[1]);
    String[] hw_days = in.readLine().split(" ");
    int sum = 0;
    for(int i=0; i<m; i++){
      sum += Integer.parseInt(hw_days[i]);
    }
    System.out.println(n-sum >= 0?n-sum:-1);
  }
}