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

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100;
const double eps = 1e-6;
const int inf = 2000000000;
const int X[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int Y[8] = {1, 1, 0, -1, -1, -1, 0, 1};
template <typename T1, typename T2>
void f(T1 a, T2 &b) {
  stringstream s;
  s << a;
  s >> b;
}
int n, a, st = -1, j = -1, d = -inf;
int ans = 1;
int kol = -1;
int main() {
  cin >> n;
  int i = 0;
  while (st == -1 && i < n) {
    cin >> st;
    j++;
    i++;
    kol++;
  }
  for (; i < n; i++) {
    cin >> a;
    if (a == -1) {
      if (d == -inf || d >= 0)
        continue;
      else {
        if (st + (i - j) * d > 0)
          continue;
        else {
          d = -inf;
          ans++;
          st = -1;
          kol = -1;
          while (st == -1 && i < n) {
            cin >> st;
            j++;
            i++;
            kol++;
          }
          j = i;
          continue;
        }
      }
    }
    if ((a - st) % (i - j) == 0) {
      if (d == -inf) {
        d = (a - st) / (i - j);
        if (st - kol * d < 1) {
          ans++;
          kol = 0;
        } else {
          kol = 0;
        }
      }
      if (st + (i - j) * d == a) continue;
    }
    d = -inf;
    ans++;
    st = a;
    j = i;
  }
  cout << ans;
  return 0;
}
