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;
template <class T>
void swap(T& a, T& b) {
  T c;
  c = a, a = b, b = c;
}
double random(double m) { return (rand() / ((double)RAND_MAX + 0.1)) * m; }
double random(double m, double n) {
  return (rand() / ((double)RAND_MAX + 0.1)) * (m - n) + n;
}
struct solve {
  vector<long long> a, b;
  long long num, inc, first, last;
  void init() {
    a.clear();
    b.clear();
    num = 0;
  }
  bool add(long long val) {
    num++;
    if (val != -1 && a.size() < 2) {
      a.push_back(val);
      b.push_back(num);
    }
    if (a.size() < 2) return true;
    if ((a[1] - a[0]) % (b[1] - b[0]) != 0) return false;
    inc = (a[1] - a[0]) / (b[1] - b[0]);
    first = a[0] - (b[0] - 1) * inc;
    last = a[0] + (num - b[0]) * inc;
    if (val != -1 && val != last) return false;
    if (first < 1 || last < 1) return false;
    return true;
  }
} s;
int main() {
  long long n, t, cnt = 0;
  cin >> n;
  s.init();
  for (int _Count_variable_ = 0; _Count_variable_ < n; _Count_variable_++) {
    cin >> t;
    if (!s.add(t)) {
      s.init();
      s.add(t);
      cnt++;
    }
  }
  if (s.num > 0) cnt++;
  cout << cnt;
  return 0;
}
