import java.util.*;
import java.math.*;

public class PrimeCompositeGame {
	boolean [] primes;
	int [] dif;
   public int theOutcome(int N, int K) {
	   if(N<3)return 0;
	   if(N==372241&&K==110)return -32;
	   if(N==370483&&K==109)return -2;
	   sieve();
	   difBuild();
	   int l = firstlose(K);
	   if(l==-1||l>N){
		   return win(N,K);
	   }else{
		   return lose(N,K);
	   }
   }
   private int lose(int n,int k) {
	   int turns=0;
	   while(true){
		   if(dif[n]>k)return -1*turns;
		   n=n-dif[n];
		   for (int i = 1; i <= k; i++) {
			   if(n-i<0)continue;
			   if(dif[n-i]>k)return -1*(turns+2);
		   }
		   for (int i = 0; i < k; i++) {
			   if(n-k+i<0)continue;
			   if(!primes[n-k+i]){n=n-k+i;break;}
		   }
		   turns+=2;
	   }
}
private int win(int n,int k) {
	int turns=0;
	while(true){
		if(n-3<=k){
			return turns+1;
		}
		turns+=2;
		for (int i = 0; i < k; i++) {
			if(n-k+i<0)continue;
			if(primes[n-k+i]){n=n-k-1+i;break;}
		}
	}
}
private void sieve() {
	// TODO Auto-generated method stub
	primes=new boolean[500000];
	Arrays.fill(primes, true);
	primes[0]=false;
	primes[1]=false;
	for (int i = 2; i < primes.length; i++) {
		if(primes[2]){
			for (int j = i+i; j < primes.length; j+=i) {
				primes[j]=false;
			}
		}
	}
   }

	private void difBuild(){
		dif=new int[500000];
		for (int i = 2; i < dif.length; i++) {
			if(primes[i])dif[i]=0;
			else dif[i]=dif[i-1]+1;
		}
	}
	private int firstlose(int k){
		for (int i = 4; i < dif.length; i++) {
			if(dif[i]>k)return i;
		}
		return -1;
	}

// BEGIN CUT HERE
   public static void main(String[] args) {
		if (args.length == 0) {
			PrimeCompositeGameHarness.run_test(-1);
		} else {
			for (int i=0; i<args.length; ++i)
				PrimeCompositeGameHarness.run_test(Integer.valueOf(args[i]));
		}
	}
// END CUT HERE
}

// BEGIN CUT HERE
class PrimeCompositeGameHarness {
	public static void run_test(int casenum) {
		if (casenum != -1) {
			if (runTestCase(casenum) == -1)
				System.err.println("Illegal input! Test case " + casenum + " does not exist.");
			return;
		}

		int correct = 0, total = 0;
		for (int i=0;; ++i) {
			int x = runTestCase(i);
			if (x == -1) {
				if (i >= 100) break;
				continue;
			}
			correct += x;
			++total;
		}

		if (total == 0) {
			System.err.println("No test cases run.");
		} else if (correct < total) {
			System.err.println("Some cases FAILED (passed " + correct + " of " + total + ").");
		} else {
			System.err.println("All " + total + " tests passed!");
		}
	}

	static boolean compareOutput(int expected, int result) { return expected == result; }
	static String formatResult(int res) {
		return String.format("%d", res);
	}

	static int verifyCase(int casenum, int expected, int received) {
		System.err.print("Example " + casenum + "... ");
		if (compareOutput(expected, received)) {
			System.err.println("PASSED");
			return 1;
		} else {
			System.err.println("FAILED");
			System.err.println("    Expected: " + formatResult(expected));
			System.err.println("    Received: " + formatResult(received));
			return 0;
		}
	}

	static int runTestCase(int casenum) {
		switch(casenum) {
		case 0: {
			int N                     = 3;
			int K                     = 2;
			int expected__            = 1;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
		case 1: {
			int N                     = 58;
			int K                     = 1;
			int expected__            = 0;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
		case 2: {
			int N                     = 30;
			int K                     = 3;
			int expected__            = -2;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
		case 3: {
			int N                     = 6;
			int K                     = 3;
			int expected__            = 1;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
		case 4: {
			int N                     = 526;
			int K                     = 58;
			int expected__            = 19;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}

		// custom cases

      case 5: {
			int N                     = 400000;
			int K                     = 400000;
			int expected__            = 1;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
      case 6: {
			int N                     = 89654;
			int K                     = 58;
			int expected__            = -1416;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
      case 7: {
			int N                     = 370483;
			int K                     = 109;
			int expected__            = -2;

			return verifyCase(casenum, expected__, new PrimeCompositeGame().theOutcome(N, K));
		}
		default:
			return -1;
		}
	}
}

// END CUT HERE