import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def SI(): return sys.stdin.readline()[:-1]

def main():
    n=II()
    l=II()
    xx=[0]+[II() for _ in range(n-1)]
    xx.reverse()
    s=0
    ans=n-1
    for i in range(n):
        if i:
            if abs(pre-xx[i])>=l:
                print(-1)
                exit()
            if (2*xx[i]-l)*i<2*s<(2*xx[i]+l)*i and (2*xx[i-1]-l)*i<2*s<(2*xx[i-1]+l)*i:ans-=1
        pre=xx[i]
        s+=xx[i]
    print(ans)

main()