import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws FileNotFoundException {
		long t = System.currentTimeMillis();
		new Main().run();
		System.err.println(System.currentTimeMillis() - t);
	}
	
	final long MOD=(long)1e9+7;
	
	void run() {
		Scanner sc = new Scanner(System.in);
		int C=sc.nextInt();
		int N=sc.nextInt();
		int[] A=new int[N];
		for(int i=0;i<N;++i) {
			A[i]=sc.nextInt();
		}
		A=Arrays.stream(A).distinct().sorted().toArray();
		N=A.length;
		// x_i>=0
		// min(sum x_i)
		// sum a_i*x_i = C
		long ans=Long.MAX_VALUE/3;
		for(int i=0;i<N;++i) {
			if(C%A[i]==0) {
				ans=Math.min(ans, C/A[i]);
			}
		}
		long[] dp=new long[C+1];
		long INF=Long.MAX_VALUE/3;
		Arrays.fill(dp, INF);
		dp[0]=0;
		for(int i=N-1;i>=0;--i) {
			for(int cur=C-1;cur>=0;--cur) {
				if((C-cur+A[i]-1)/A[i]>=ans)continue;
				for(int add=1;(long)add*A[i]+cur<=C;++add) {
					dp[add*A[i]+cur]=Math.min(dp[add*A[i]+cur], dp[cur]+add);
					if(dp[cur]+add>=ans)break;
				}
				ans=Math.min(ans, dp[C]);
			}
		}
		System.out.println(ans==INF?-1:ans);
	}
	
	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}

}
	
	
class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    private void skipUnprintable() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;}
    public boolean hasNext() { skipUnprintable(); return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    
    public int nextInt() {
    	return (int)nextLong();
    }
}