n,*x = map(int,open(0).read().split())
L = x[0]
g = cnt = x[0] = 0
ans = n-1
for i in range(n-2,-1,-1):
    cnt += 1
    g += x[i+1]
    if 2*cnt*max(x[i],x[i+1]) < 2*g+L*cnt < 2*cnt*(min(x[i],x[i+1])+L):
        ans -= 1
    if abs(x[i+1]-x[i]) >= L:
        ans = -1
print(max(ans,-1))