import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Iterator;


public class Main_yukicoder343 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        Printer pr = new Printer(System.out);

        int n = sc.nextInt();
        int l = sc.nextInt();

        int[] x = new int[n];
        for (int i = 1; i < n; i++) {
        	x[i] = sc.nextInt();
        }

        if (n == 1) {
        	pr.println(0);
        } else {
        	double h = (double)l / 2;
        	double g = x[n - 1];
        	int ret = 0;
        	for (int i = n - 2; i >= 0; i--) {
        		if (Math.abs(x[i + 1] - x[i]) >= l) {
        			ret = -1;
        			break;
        		} else if ((g <= (double)x[i] - h || g >= (double)x[i] + h) || (g <= (double)x[i + 1] - h || g >= (double)x[i + 1] + h)) {
        			ret++;
        		}
        		g = (g * (n - 1 - i) + x[i]) / (n - 1 - i + 1);
        	}

        	pr.println(ret);
        }

        pr.close();
        sc.close();
    }

	@SuppressWarnings("unused")
	private static class Scanner {
		BufferedReader br;
		Iterator<String> it;

		Scanner (InputStream in) {
			br = new BufferedReader(new InputStreamReader(in));
		}

		String next() throws RuntimeException  {
			try {
				if (it == null || !it.hasNext()) {
					it = Arrays.asList(br.readLine().split(" ")).iterator();
				}
				return it.next();
			} catch (IOException e) {
				throw new IllegalStateException();
			}
		}

		int nextInt() throws RuntimeException {
			return Integer.parseInt(next());
		}

		long nextLong() throws RuntimeException {
			return Long.parseLong(next());
		}

		float nextFloat() throws RuntimeException {
			return Float.parseFloat(next());
		}

		double nextDouble() throws RuntimeException {
			return Double.parseDouble(next());
		}

		void close() {
			try {
				br.close();
			} catch (IOException e) {
//				throw new IllegalStateException();
			}
		}
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}