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().strip())
ais = map(int, raw_input().strip().split(' '))

# n = 9
# ais = [-1, 6, -1, 2, -1, 4, 7, -1, 2]

condensed = []
cvals = None
ctype = None

for ai in ais:
    if ctype is None:
        if ai == -1:
            ctype = "G"
            cvals = 1
        else:
            ctype = "S"
            cvals = [ai]
    elif ctype == "G":
        if ai == -1:
            cvals += 1
        else:
            condensed.append((ctype, cvals))
            ctype = "S"
            cvals = [ai]
    else:  # ctype == "S"
        if ai == -1:
            condensed.append((ctype, cvals))
            ctype = "G"
            cvals = 1
        elif len(cvals) == 1:
            cvals.append(ai)
        else:
            if cvals[-1] - cvals[-2] == ai - cvals[-1]:
                cvals.append(ai)
            else:
                condensed.append((ctype, tuple(cvals)))
                cvals = [ai]

if ctype == "S":
    condensed.append((ctype, tuple(cvals)))
else:
    condensed.append((ctype, cvals))

# print "condensed: %s" % condensed


def close(val):
    global total
    global diff
    total += 1

    if len(val) > 1:
        diff = val[-1] - val[-2]
    else:
        diff = None


total = 0
diff = None

if len(condensed) == 1:
    print 1
elif len(condensed) == 2:
    if condensed[0][0] != condensed[1][0]:  # S G or G S
        print 1
    else:  # S S
        print 2
else:
    for i, (ctype, val) in enumerate(condensed[1:], 1):
        # print "i: %s, ctype: %s, val: %s" % (i, ctype, val)
        if ctype == "S":
            # print "diff: %s" % diff
            if condensed[i - 1][0] == "S":
                # print "A"
                close(val)
                # print "close at: %s" % i
            else:
                if diff is not None:
                    # print "B"
                    gap_len = condensed[i - 1][1]
                    last_val = condensed[i - 2][1][-1]
                    if last_val + diff * (gap_len + 1) != val[0]:
                        # print "close at: %s" % i
                        close(val)
                    elif len(val) > 1 and val[1] - val[0] != diff:
                        close(val)
                        # print "close at: %s" % i

                else:
                    # print "C"
                    if i > 1:
                        gap_len = condensed[i - 1][1]
                        last_val = condensed[i - 2][1][-1]
                        if len(val) == 1:
                            if (val[0] - last_val) % (gap_len + 1) != 0:
                                close(val)
                                # print "close at %s" % i
                            else:
                                diff = (val[0] - last_val) / (gap_len + 1)
                        else:
                            diff = val[1] - val[0]
                            if last_val + diff * (gap_len + 1) != val[0]:
                                close(val)
                                # print "close at: %s" % i

    # print "close at: %s" % len(condensed)
    total += 1
    print total





