#!/usr/bin/env python3.8
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import numpy as np


# %%
N = int(readline())
L = int(readline())
X = np.array([0] + read().split(), np.int64)

# %%
X = X[::-1]


# %%
def solve(X):
    if np.any(np.abs(np.diff(X)) >= L):
        return - 1
    Xcum = np.cumsum(X)
    M = (2 * Xcum + np.arange(1, N + 1) * L)[:-1]
    LHS = np.maximum(2 * X[:-1], 2 * X[1:]) * np.arange(1, N)
    RHS = np.minimum(2 * X[:-1], 2 * X[1:]) * np.arange(1, N) + np.arange(1, N) * L * 2
    omit = (LHS < M) & (M < RHS)
    return N - 1 - np.count_nonzero(omit)


# %%
print(solve(X))