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