import sys
input = sys.stdin.readline

N=int(input())
L=int(input())

X=[[0,L]]
S=[]

for i in range(N-1):
    x=int(input())
    S.append(x)
    X.append([x,x+L])

for i in range(N-1):
    x,y=X[i]
    z,w=X[i+1]

    if y<=z or w<=x:
        print(-1)
        exit()

for i in range(len(S)-2,-1,-1):
    S[i]+=S[i+1]
    
ANS=0

for i in range(N-1):
    x,y=X[i]
    z,w=X[i+1]

    k=S[i]/(N-1-i)+L/2

    if x<k<y and z<k<w:
        pass
    else:
        ANS+=1
print(ANS)