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>
inline void CLR(T &g) {
  T t;
  swap(t, g);
}
template <class T>
inline void CLR(T &g, long long a) {
  memset(g, a, sizeof g);
}
template <class T>
inline bool inr(T a, T b, T c) {
  return (a >= b && a <= c);
}
template <class T>
inline bool odd(T a) {
  return a & 1;
}
template <class T>
inline bool empty(T a, T b) {
  return !(a & (1 << (b - 1)));
}
template <class T>
inline void bin(T &a, T b) {
  a |= 1 << (b - 1);
}
inline long long read() {
  char ch = getchar();
  long long tmp = 0;
  for (; ch > '9' || ch < '0'; ch = getchar())
    ;
  for (; '0' <= ch && ch <= '9'; ch = getchar())
    tmp = tmp * 10 + (long long)ch - 48;
  return tmp;
}
long long N, M, K, Q;
long long a[200005];
long long f[200005], g[200005], h[200005];
int main() {
  long long T, o = 0;
  cin >> N;
  for (long long(i) = 1; (i) <= (N); (i)++) cin >> a[i];
  f[1] = 1;
  g[1] = -0x3f3f3f3f;
  if (a[1] > 0)
    h[1] = 1;
  else
    h[1] = -1;
  for (long long(i) = 2; (i) <= (N); (i)++) {
    if (a[i] == -1 && g[i - 1] != -0x3f3f3f3f &&
        (a[f[i - 1]] + (i - f[i - 1]) * g[i - 1] <= 0 ||
         a[f[i - 1]] + (i - f[i - 1]) * g[i - 1] >= 1000000000)) {
      f[i] = i;
      g[i] = -0x3f3f3f3f;
      h[i] = -1;
      continue;
    }
    if (a[i] == -1) {
      f[i] = f[i - 1];
      g[i] = g[i - 1];
      h[i] = h[i - 1];
    }
    if (a[i] != -1 && g[i - 1] != -0x3f3f3f3f) {
      if (a[f[i - 1]] + (i - f[i - 1]) * g[i - 1] != a[i]) {
        f[i] = i;
        g[i] = -0x3f3f3f3f;
        h[i] = i;
        continue;
      } else if (a[i] - (i - f[i - 1]) * (g[i - 1]) <= 0 ||
                 a[i] - (i - f[i - 1]) * (g[i - 1]) >= 1000000000) {
        f[i] = i;
        g[i] = -0x3f3f3f3f;
        h[i] = i;
      } else {
        f[i] = f[i - 1];
        g[i] = g[i - 1];
        h[i] = i;
        continue;
      }
    }
    if (a[i] != -1 && g[i - 1] == -0x3f3f3f3f) {
      if (h[i - 1] == -1) {
        f[i] = f[i - 1];
        g[i] = -0x3f3f3f3f;
        h[i] = i;
        continue;
      } else {
        long long t = a[i] - a[h[i - 1]];
        if (t % (i - h[i - 1]) == 0) {
          f[i] = f[i - 1];
          g[i] = t / (i - f[i - 1]);
          h[i] = i;
          continue;
        } else {
          f[i] = i;
          g[i] = -0x3f3f3f3f;
          h[i] = i;
          continue;
        }
      }
    }
  }
  int ans = 0;
  for (long long(i) = 1; (i) <= (N); (i)++)
    if (f[i] == i) ans++;
  printf("%d\n", ans);
  return 0;
}
