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 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); } } }