import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.InputStream; import java.io.PrintWriter; import java.util.Arrays; import java.util.InputMismatchException; import java.io.OutputStream; import java.util.StringTokenizer; import java.io.BufferedReader; import java.io.InputStreamReader; import java.util.HashMap; import java.util.Map; import java.util.ArrayList; import java.util.List; import java.util.Scanner; import java.math.BigDecimal; /** * Built using CHelper plug-in * Actual solution is at the top */ public class Main { public static void main(String[] args) { InputStream inputStream = System.in; OutputStream outputStream = System.out; InputReader in = new InputReader(inputStream); PrintWriter out = new PrintWriter(outputStream); Task solver = new Task(); solver.solve(1, in, out); out.close(); } static class Task { public void solve(int testNumber, InputReader in, PrintWriter out) { int n = in.nextInt(); int m = in.nextInt(); if(n/m==2&&n%m==0) out.println(2); else if(n<=m) out.println(1); else out.println(-1); } } static class InputReader { public BufferedReader reader; public StringTokenizer tokenizer; public InputReader(InputStream stream) { reader = new BufferedReader(new InputStreamReader(stream), 32768); tokenizer = null; } public String next() { while (tokenizer == null || !tokenizer.hasMoreTokens()) { try { tokenizer = new StringTokenizer(reader.readLine()); } catch (IOException e) { throw new RuntimeException(e); } } return tokenizer.nextToken(); } public int nextInt() { return Integer.parseInt(next()); } public long nextLong() { return Long.parseLong(next()); } } }