import sys

def main():
    N = int(sys.stdin.readline())
    L = int(sys.stdin.readline())
    if N == 1:
        print(0)
        return
    
    X = [0] * (N + 2)  # 1-based index, X[1] = 0
    for i in range(2, N+1):
        X[i] = int(sys.stdin.readline())
    
    # Check contact area for all adjacent plates
    all_contact_ok = True
    for i in range(1, N):
        a1 = X[i]
        b1 = a1 + L
        a2 = X[i+1]
        b2 = a2 + L
        overlap = max(0, min(b1, b2) - max(a1, a2))
        if overlap < 1:
            all_contact_ok = False
            break
    
    if not all_contact_ok:
        print(-1)
        return
    
    # Precompute suffix sums for X
    sum_suffix = [0] * (N + 2)
    sum_suffix[N] = X[N]
    for i in range(N-1, 0, -1):
        sum_suffix[i] = X[i] + sum_suffix[i+1]
    
    ans = 0
    for i in range(1, N):
        sum_x = sum_suffix[i+1]
        m = N - i
        x_g = (sum_x / m) + L / 2.0
        
        a_i = X[i]
        b_i = a_i + L
        a_j = X[i+1]
        b_j = a_j + L
        
        if not (a_i < x_g < b_i and a_j < x_g < b_j):
            ans += 1
    
    print(ans)

if __name__ == '__main__':
    main()