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[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } else {
      System.out.println(-1);
    }
  }
} 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[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } 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[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } else {
      System.out.println(-1);
    }
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int M = sc.nextInt();
    int[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } 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[] A = new int[M]; A new int[M] M int total =0; total 0 for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    } int i =0; int i =0; i 0 i<M i M i++ i++ i {
      A[i] = sc.nextInt();
      total += A[i];
    } {
      A[i] = sc.nextInt();
      total += A[i];
    } A[i] = sc.nextInt() A[i] A i sc.nextInt() sc.nextInt sc total += A[i] total A[i] A i if (N - total >= 0) {
      System.out.println(N - total);
    } else {
      System.out.println(-1);
    } N - total >= 0 N - total N total 0 {
      System.out.println(N - total);
    } System.out.println(N - total) System.out.println System.out System System.out N - total N total {
      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[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } 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[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } else {
      System.out.println(-1);
    }
  }
} 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[] A = new int[M];
    int total =0;
    for (int i =0; i<M; i++) {
      A[i] = sc.nextInt();
      total += A[i];
    }
    if (N - total >= 0) {
      System.out.println(N - total);
    } else {
      System.out.println(-1);
    }
  }
}