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)
chainCount = 0
chainStarted = False
chainPrevIndex = -1
chainStep = None
potentialMinuses = 0
for i in xrange(n):
    if a[i] == -1:
        if i == n-1 and chainStep != None and chainStep < 0 and a[chainPrevIndex] + chainStep*( i - chainPrevIndex ) <= 0:
            chainCount += 1
        elif not chainStarted:
            potentialMinuses += 1
        continue
    if not chainStarted:
        chainStarted = True
        chainPrevIndex = i
        chainStep = None
    else:
        # check if current element is in the chain
        steps = i - chainPrevIndex
        if chainStep == None:
            if (a[i] - a[chainPrevIndex]) % steps:
                chainCount += 1 # end previous chain and create new
                potentialMinuses = 0
            else: # we are still in the chain
                chainStep = (a[i] - a[chainPrevIndex]) / steps
                if a[chainPrevIndex] - chainStep*potentialMinuses <= 0:
                    chainCount += 1
                    chainStep = None
                potentialMinuses = 0
        else:
            if a[i] - a[chainPrevIndex] == steps*chainStep: # we are still in the chain
                pass
            else: 
                chainCount += 1
                # check if we need additional chain for -1s:
                if chainStep < 0 and i - chainPrevIndex > 1 and a[chainPrevIndex] + (i - chainPrevIndex - 1)*chainStep <= 0: # there are -1 in between which are <= 0
                    potentialMinuses = i - chainPrevIndex - 1 - ( a[chainPrevIndex] / -chainStep )                    
                else: 
                    potentialMinuses = 0
                chainStep = None
        chainPrevIndex = i
    #print i, a[i], potentialMinuses, chainCount
chainCount += 1



print chainCount