w,h,*b = map(int,open(0).read().split())

M = 2**15
from collections import deque
q = deque()
d = {}
for i in [1,w]:
    if b[0] != b[i]:
        q.append(i*M)
        d[i*M] = 1

def check(c,x2,delta):
    if c*M+x2 in d: return
    if delta*(b[c]-b[x2]) < 0:
        q.append(c*M+x2)
        d[c*M+x2] = d[x] + 1

while q:
    x = q.popleft()
    x1,x2 = x%M, x//M
    h2,w2 = x2//w, x2%w
    delta = b[x2] - b[x1]
    if h2: check(x2-w,x2,delta)
    if h2+1 < h: check(x2+w,x2,delta)
    if w2: check(x2-1,x2,delta)
    if w2+1 < w: check(x2+1,x2,delta)

ans = INF = 10**9
for c in [(w*h-1)*M + (w*h-2), (w*h-1)*M + (w*h-1-w)]:
    if c in d:
        ans = min(ans,d[c])
print(ans if ans != INF else -1)