3
-1 1 -1
 4
45 -1 41 -1
 1
-1
 5
40 -1 44 46 48
 6
43 40 37 34 -1 -1
 7
-1 2 4 -1 4 1 5
 19
23 26 -1 -1 35 38 41 -1 -1 -1 53 -1 59 62 6 7 8 9 -1
 6
-1 2 6 -1 -1 6
 16
3 8 13 18 23 -1 -1 -1 43 48 53 45 -1 -1 -1 -1
 13
25 24 23 22 24 27 -1 33 -1 2 2 2 -1
 12
-1 17 -1 54 -1 64 -1 74 79 84 -1 94
 8
-1 12 14 16 18 20 -1 -1
 13
2 -1 3 1 3 1 -1 1 3 -1 -1 1 1
 5
-1 40 42 -1 46
 2
1000000000 -1
 14
-1 5 3 -1 -1 31 31 31 -1 31 -1 -1 4 7
 3
-1 1000000000 999999999
 7
32 33 34 -1 -1 37 38
 1
1000000000
 10
29 27 -1 23 42 -1 -1 45 -1 -1
 3
39 42 -1
 15
-1 28 -1 32 34 26 -1 26 -1 -1 26 26 26 -1 -1
 3
1000000000 999999999 1000000000
 17
-1 -1 -1 -1 64 68 72 -1 45 46 47 48 49 50 51 52 53
 3
-1 1 1000000000
 2
-1 1000000000
 3
999999999 -1 1000000000
 1
1
 2
1000000000 1000000000
 3
999999999 1000000000 -1
 7
11 8 5 -1 -1 -1 -1
 3
-1 1 2
 2
-1 21
 8
-1 -1 1 7 -1 9 5 2
 11
9 21 17 13 -1 -1 -1 -1 -1 -1 -1
 3
-1 1000000000 -1
 6
-1 6 1 -1 -1 -1
 1
65
 20
-1 32 37 -1 -1 -1 57 -1 -1 40 31 33 -1 -1 39 47 43 -1 35 32
 5
-1 1 7 -1 5
 18
21 19 -1 -1 -1 48 50 -1 54 -1 5 1 -1 -1 -1 37 36 35
 9
42 39 36 33 -1 -1 -1 34 39
 2
-1 -1

1
 1
 1
 1
 1
 3
 2
 2
 2
 3
 2
 1
 6
 1
 1
 3
 1
 1
 1
 2
 1
 2
 2
 2
 2
 1
 2
 1
 1
 1
 2
 2
 1
 3
 3
 1
 2
 1
 5
 2
 4
 2
 1

#################
n = int( raw_input() )

a = map(int, raw_input().split() )

#n = len(a)
#print a
chainCount = 1
chainPrevIndex = None
chainStep = None
potentialMinuses = 0
####################

def fillMinuses(j, a, chainCount, chainPrevIndex, chainStep, potentialMinuses):
    chain42 = False
    while potentialMinuses > 0:
        if a[j] == -1:
            if chainStep == 0:
                a[j] = a[chainPrevIndex]
            else:
                newVal = a[j+1] - chainStep
                if not chain42 and newVal > 0:
                    a[j] = newVal
                else:
                    chain42 = True
                    a[j] = 42
            potentialMinuses -= 1
        j -= 1
    if chain42:
        chainCount += 1
    return chainCount

if n == 1:
    print 1
    exit()


for i in xrange(n):    
    if a[i] == -1:
        if chainStep == None:
            potentialMinuses += 1
        else:
            assert(potentialMinuses == 0)
            newVal = a[i-1] + chainStep
            if newVal > 0:
                a[i] = newVal
                chainPrevIndex = i
            else: # this -1 cannot be part of previous chain
                chainCount +=1
                chainPrevIndex = None
                chainStep = None
                potentialMinuses = 1
    else:
        if chainPrevIndex == None:
            chainPrevIndex = i
        elif chainStep == None:
            # check if current element is in the current chain
            if (a[i] - a[chainPrevIndex]) % (i - chainPrevIndex) == 0:
                chainStep = (a[i] - a[chainPrevIndex]) / (i - chainPrevIndex)
                chainCount = fillMinuses(i-1, a, chainCount, chainPrevIndex, chainStep, potentialMinuses)
                potentialMinuses = 0
                chainPrevIndex = i
            else:
                chainStep = 0
                chainCount = fillMinuses(i-1, a, chainCount, chainPrevIndex, chainStep, potentialMinuses)
                potentialMinuses = 0
                chainStep = None
                chainPrevIndex = i
        else: 
            if a[i-1] + chainStep == a[i]:
                chainPrevIndex = i
            else:
                chainCount +=1
                chainPrevIndex = i
                chainStep = None

if potentialMinuses > 0:
    if a[n-1] == -1:
        a[n-1] = 42 if chainPrevIndex == None else a[chainPrevIndex]
        chainPrevIndex = n-1
        potentialMinuses -= 1
    chainStep = 0
    chainCount = fillMinuses(n-2, a, chainCount, chainPrevIndex, chainStep, potentialMinuses)
    potentialMinuses = 0
#print a
print chainCount
