Vova's house is an array consisting of n elements (yeah, this is the first problem, I think, where someone lives in the array). There are heaters in some positions of the array. The i-th element of the array is 1 if there is a heater in the position i, otherwise the i-th element of the array is 0.

Each heater has a value r (r is the same for all heaters). This value means that the heater at the position pos can warm up all the elements in range [pos - r + 1; pos + r - 1].

Vova likes to walk through his house while he thinks, and he hates cold positions of his house. Vova wants to switch some of his heaters on in such a way that each element of his house will be warmed up by at least one heater. 

Vova's target is to warm up the whole house (all the elements of the array), i.e. if n = 6, r = 2 and heaters are at positions 2 and 5, then Vova can warm up the whole house if he switches all the heaters in the house on (then the first 3 elements will be warmed up by the first heater and the last 3 elements will be warmed up by the second heater).

Initially, all the heaters are off.

But from the other hand, Vova didn't like to pay much for the electricity. So he wants to switch the minimum number of heaters on in such a way that each element of his house is warmed up by at least one heater.

Your task is to find this number of heaters or say that it is impossible to warm up the whole house.

Input

The first line of the input contains two integers n and r (1 ≤ n, r ≤ 1000) — the number of elements in the array and the value of heaters.

The second line contains n integers a_1, a_2, ..., a_n (0 ≤ a_i ≤ 1) — the Vova's house description.

Output

Print one integer — the minimum number of heaters needed to warm up the whole house or -1 if it is impossible to do it.

Examples

Input

6 2
0 1 1 0 0 1


Output

3


Input

5 3
1 0 0 0 1


Output

2


Input

5 10
0 0 0 0 0


Output

-1


Input

10 3
0 0 1 1 0 1 0 0 0 1


Output

3

Note

In the first example the heater at the position 2 warms up elements [1; 3], the heater at the position 3 warms up elements [2, 4] and the heater at the position 6 warms up elements [5; 6] so the answer is 3.

In the second example the heater at the position 1 warms up elements [1; 3] and the heater at the position 5 warms up elements [3; 5] so the answer is 2.

In the third example there are no heaters so the answer is -1.

In the fourth example the heater at the position 3 warms up elements [1; 5], the heater at the position 6 warms up elements [4; 8] and the heater at the position 10 warms up elements [8; 10] so the answer is 3.
import java.util.Arrays;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import java.util.Random;
import java.io.FileWriter;
import java.io.PrintWriter;
/*
    Solution Created: 23:58:04 17/01/2021
    Custom Competitive programming helper.
*/

public class Main {
	
	public static void solve() {
		int n = in.nextInt(), k = in.nextInt()-1;
		int[] a = in.na(n);
		int ans = 0;
		boolean can = true;
		int[] closestPrev = new int[n];
		closestPrev[0] = a[0]==1?0:-100000000;
		for(int i = 1; i<n; i++) {
			closestPrev[i] = closestPrev[i-1];
			if(a[i]==1) closestPrev[i] = i;
		}
		int i = 0;
		while(i<n) {
			int toTake = closestPrev[Math.min(n-1, i+k)];
			if(Math.abs(toTake-i)>k) {
				can = false;
				break;
			}
			ans++;
			i = toTake+k+1;
		}
		out.println(can?ans:-1);
	}
	
	public static void main(String[] args) {
		in = new Reader();
		out = new Writer();
		int t = 1;
		while(t-->0) solve();
		out.exit();
	}
	static Reader in; static Writer out;

static class Reader {
	static BufferedReader br;
	static StringTokenizer st;
	
	public Reader() {
		this.br = new BufferedReader(new InputStreamReader(System.in));
	}
	
	public Reader(String f){
		try {
			this.br = new BufferedReader(new FileReader(f));
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	public int[] na(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++) a[i] = nextInt();
		return a;
	}

	public double[] nd(int n) {
		double[] a = new double[n];
		for (int i = 0; i < n; i++) a[i] = nextDouble();
		return a;
	}
	
	public long[] nl(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++) a[i] = nextLong();
		return a;
	}

	public char[] nca() {
		return next().toCharArray();
	}

	public String[] ns(int n) {
		String[] a = new String[n];
		for (int i = 0; i < n; i++) a[i] = next();
		return a;
	}

	public int nextInt() {
		ensureNext();
		return Integer.parseInt(st.nextToken());
	}

	public double nextDouble() {
		ensureNext();
		return Double.parseDouble(st.nextToken());
	}

	public Long nextLong() {
		ensureNext();
		return Long.parseLong(st.nextToken());
	}

	public String next() {
		ensureNext();
		return st.nextToken();
	}
	
	public String nextLine() {
		try {
			return br.readLine();
		} catch (Exception e) {
			e.printStackTrace();
			return null;
		}
	}
	
	private void ensureNext() {
		if (st == null || !st.hasMoreTokens()) {
			try {
				st = new StringTokenizer(br.readLine());
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

static class Util{
		private static Random random = new Random();
		static long[] fact;
		
		public static void initFactorial(int n, long mod) {
			fact = new long[n+1];
			fact[0] = 1;
			for (int i = 1; i < n+1; i++) fact[i] = (fact[i - 1] * i) % mod;
		}
		
		public static long modInverse(long a, long MOD) {
			long[] gcdE = gcdExtended(a, MOD);
			if (gcdE[0] != 1) return -1; // Inverted doesn't exist
			long x = gcdE[1];
			return (x % MOD + MOD) % MOD;
		}
		
		public static long[] gcdExtended(long p, long q) {
			if (q == 0) return new long[] { p, 1, 0 };
			long[] vals = gcdExtended(q, p % q);
			long tmp = vals[2];
			vals[2] = vals[1] - (p / q) * vals[2];
			vals[1] = tmp;
			return vals;
		}
		
		public static long nCr(int n, int r, long MOD) {
			if (r == 0) return 1;
			return (fact[n] * modInverse(fact[r], MOD) % MOD * modInverse(fact[n - r], MOD) % MOD) % MOD;
		}
		
		public static long nCr(int n, int r) {
			return (fact[n]/fact[r])/fact[n-r];
		}
		
		public static long nPr(int n, int r, long MOD) {
			if (r == 0) return 1;
			return (fact[n] * modInverse(fact[n - r], MOD) % MOD) % MOD;
		}
		public static long nPr(int n, int r) {
			return fact[n]/fact[n-r];
		}
		
		public static boolean isPrime(int n) { 
	        if (n <= 1) return false; 
	        if (n <= 3) return true; 
	        if (n % 2 == 0 || n % 3 == 0) return false; 
	        for (int i = 5; i * i <= n; i = i + 6) 
	            if (n % i == 0 || n % (i + 2) == 0) 
	            return false; 
	        return true; 
	    }
		
	    public static boolean[] getSieve(int n) {
	        boolean[] isPrime = new boolean[n+1];
	        for (int i = 2; i <= n; i++) isPrime[i] = true;
	        for (int i = 2; i*i <= n; i++) if (isPrime[i]) 
	                for (int j = i; i*j <= n; j++) isPrime[i*j] = false;
	        return isPrime;
	    }
	    
	    public static int gcd(int a, int b) { 
	    	int tmp = 0;
	    	while(b != 0) {
	    		tmp = b;
	    		b = a%b;
	    		a = tmp;
	    	}
	    	return a;
	    }
	    
	    public static long gcd(long a, long b) { 
	    	long tmp = 0;
	    	while(b != 0) {
	    		tmp = b;
	    		b = a%b;
	    		a = tmp;
	    	}
	    	return a;
	    }
	    
	    public static int random(int min, int max) {
	    	return random.nextInt(max-min+1)+min;
	    }
	    
		public static void dbg(Object... o) { 
			System.out.println(Arrays.deepToString(o)); 
		}
		
		public static void reverse(int[] s, int l , int r) {
			for(int i = l; i<=(l+r)/2; i++) {
				int tmp = s[i];
				s[i] = s[r+l-i];
				s[r+l-i] = tmp;
			}
	    }
		
		public static void reverse(int[] s) {
			reverse(s, 0, s.length-1);
	    }
		
		public static void reverse(long[] s, int l , int r) {
			for(int i = l; i<=(l+r)/2; i++) {
				long tmp = s[i];
				s[i] = s[r+l-i];
				s[r+l-i] = tmp;
			}
	    }
		
		public static void reverse(long[] s) {
			reverse(s, 0, s.length-1);
	    }
		
		public static void reverse(float[] s, int l , int r) {
			for(int i = l; i<=(l+r)/2; i++) {
				float tmp = s[i];
				s[i] = s[r+l-i];
				s[r+l-i] = tmp;
			}
	    }
		
		public static void reverse(float[] s) {
			reverse(s, 0, s.length-1);
	    }
		
		public static void reverse(double[] s, int l , int r) {
			for(int i = l; i<=(l+r)/2; i++) {
				double tmp = s[i];
				s[i] = s[r+l-i];
				s[r+l-i] = tmp;
			}
	    }
		
		public static void reverse(double[] s) {
			reverse(s, 0, s.length-1);
	    }
		
		public static void reverse(char[] s, int l , int r) {
			for(int i = l; i<=(l+r)/2; i++) {
				char tmp = s[i];
				s[i] = s[r+l-i];
				s[r+l-i] = tmp;
			}
	    }
		
		public static void reverse(char[] s) {
			reverse(s, 0, s.length-1);
	    }
		
		public static <T> void reverse(T[] s, int l , int r) {
			for(int i = l; i<=(l+r)/2; i++) {
				T tmp = s[i];
				s[i] = s[r+l-i];
				s[r+l-i] = tmp;
			}
	    }
		
		public static <T> void reverse(T[] s) {
			reverse(s, 0, s.length-1);
	    }
		
		public static void shuffle(int[] s) {
	        for (int i = 0; i < s.length; ++i) {
	            int j = random.nextInt(i + 1);
	            int t = s[i];
	            s[i] = s[j];
	            s[j] = t;
	        }
	    }
		
	    public static void shuffle(long[] s) {
	        for (int i = 0; i < s.length; ++i) {
	            int j = random.nextInt(i + 1);
	            long t = s[i];
	            s[i] = s[j];
	            s[j] = t;
	        }
	    }
	    
	    public static void shuffle(float[] s) {
	        for (int i = 0; i < s.length; ++i) {
	            int j = random.nextInt(i + 1);
	            float t = s[i];
	            s[i] = s[j];
	            s[j] = t;
	        }
	    }
	    
	    public static void shuffle(double[] s) {
	        for (int i = 0; i < s.length; ++i) {
	            int j = random.nextInt(i + 1);
	            double t = s[i];
	            s[i] = s[j];
	            s[j] = t;
	        }
	    }
	    
	    public static void shuffle(char[] s) {
	        for (int i = 0; i < s.length; ++i) {
	            int j = random.nextInt(i + 1);
	            char t = s[i];
	            s[i] = s[j];
	            s[j] = t;
	        }
	    }
	    
	    public static <T> void shuffle(T[] s) {
	        for (int i = 0; i < s.length; ++i) {
	            int j = random.nextInt(i + 1);
	            T t = s[i];
	            s[i] = s[j];
	            s[j] = t;
	        }
	    }
	    
		public static void sortArray(int[] a) {
	        shuffle(a);
	        Arrays.sort(a);
	    }
		
		public static void sortArray(long[] a) {
			shuffle(a);
	        Arrays.sort(a);
	    }
		
		public static void sortArray(float[] a) {
			shuffle(a);
	        Arrays.sort(a);
	    }
		
		public static void sortArray(double[] a) {
			shuffle(a);
	        Arrays.sort(a);
	    }
		
		public static void sortArray(char[] a) {
			shuffle(a);
	        Arrays.sort(a);
	    }
		
		public static <T extends Comparable<T>> void sortArray(T[] a) {
	        Arrays.sort(a);
	    }
	}

static class Writer {
	private PrintWriter pw;
	public Writer(){
		pw = new PrintWriter(System.out);
	}
	
	public Writer(String f){
		try {
			pw = new PrintWriter(new FileWriter(f));
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	
	public void printArray(int[] a) {
		for(int i = 0; i<a.length; i++) print(a[i]+" ");
	}
	
	public void printlnArray(int[] a) {
		for(int i = 0; i<a.length; i++) print(a[i]+" ");
		pw.println();
	}
	
	public void printArray(long[] a) {
		for(int i = 0; i<a.length; i++) print(a[i]+" ");
	}
	
	public void printlnArray(long[] a) {
		for(int i = 0; i<a.length; i++) print(a[i]+" ");
		pw.println();
	}
	
	public void print(Object o) {
		pw.print(o.toString());
	}
	
	public void println(Object o) {
		pw.println(o.toString());
	}
	
	public void println() {
		pw.println();
	}
	
	public void flush() {
		pw.flush();
	}
	public void exit() {
		pw.close();
	}
}
}
