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;
inline void read(int &x) {
  short negative = 1;
  x = 0;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') negative = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    x = (x << 3) + (x << 1) + (c ^ 48), c = getchar();
  x *= negative;
}
inline void write(long long x) {
  static long long sta[35];
  long long top = 0;
  do {
    sta[top++] = x % 10, x /= 10;
  } while (x);
  while (top) putchar(sta[--top] + 48);
  puts("");
}
bool prime(int n) {
  if (n == 0 || n == 1) return false;
  for (int i = 2; i * i <= n; i++)
    if (n % i == 0) return false;
  return true;
}
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};
int a[200010];
pair<long long, long long> p[200010];
long long INF = 1e10;
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < (int)(n); i++) {
    cin >> a[i];
  }
  int last = 0;
  int i;
  for (i = 0; i < n; i++) {
    if (a[i] != -1) {
      last = i;
      break;
    }
  }
  if (n - i < 2) {
    if (a[n - 1] - n + 1 > 0 || a[n - 1] == -1) {
      cout << 1 << endl;
    } else {
      cout << 2 << endl;
    }
    return 0;
  }
  for (int j = i; j <= n; j++) {
    if (a[j] != -1) {
      p[j] = pair<long long, long long>(a[last] - a[j], j - last);
      last = j;
    }
  }
  for (int i = 1; i < n; i++) {
    if (p[i].second != 0) {
      if (p[i].first % p[i].second == 0)
        p[i].first /= p[i].second;
      else if (p[i - 1].second != 0) {
        p[i - 1].first = p[i].first;
      }
    } else {
      if (p[i - 1].second != 0) {
        p[i - 1].first = p[i].first;
      }
    }
  }
  int ans = 0;
  for (int i = 1; i < n; i++) {
    if (p[i] != p[i - 1]) ans++, i++;
  }
  cout << max(ans, 1) << endl;
  return 0;
}
