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;
ifstream fin("A.in");
ofstream fout("A.out");
int n, a[200001], v[200001], fron[200001], behind[200001], progr[200001], t;
bool correct(long long x) {
  if (1 <= x && x <= 1000000000) return 1;
  return 0;
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    cin >> a[i];
  }
  int cnt = 0;
  for (int i = 1; i <= n; ++i) {
    if (a[i] == -1) {
      ++cnt;
    } else {
      v[++t] = i;
      behind[i] = cnt;
      cnt = 0;
    }
  }
  for (int i = n; i >= 1; --i) {
    if (a[i] == -1) {
      ++cnt;
    } else {
      fron[i] = cnt;
      cnt = 0;
    }
  }
  if (t == 0) {
    cout << 1;
    return 0;
  }
  cnt = 1;
  progr[1] = 0;
  for (int i = 2; i <= t; ++i) {
    if (!progr[i - 1]) {
      if ((a[v[i]] - a[v[i - 1]]) % (v[i] - v[i - 1]) == 0) {
        int r = (a[v[i]] - a[v[i - 1]]) / (v[i] - v[i - 1]);
        int ok = 0;
        if (i > 2) {
          if (!progr[i - 2]) ok = 1;
          if (!ok) {
            for (int j = v[i - 2]; j < v[i - 1]; ++j) {
              if (correct(a[v[i - 2]] + 1LL * (j - v[i - 2]) * progr[i - 2]) &&
                  correct(a[v[i - 1]] - 1LL * (v[i - 1] - j - 1) * r) > 0) {
                ok = 1;
                break;
              }
            }
          }
        } else
          ok = (correct(a[v[i - 1]] - 1LL * r * behind[v[i - 1]]));
        if (ok) {
          progr[i] = r;
        } else
          progr[i] = 0, ++cnt;
      } else
        progr[i] = 0, ++cnt;
    } else {
      if ((a[v[i]] - a[v[i - 1]]) % (v[i] - v[i - 1]) == 0) {
        int r = (a[v[i]] - a[v[i - 1]]) / (v[i] - v[i - 1]);
        if (r == progr[i - 1]) {
          progr[i] = r;
        } else
          progr[i] = 0, ++cnt;
      } else
        progr[i] = 0, ++cnt;
    }
  }
  if (progr[t]) {
    if (!correct(a[v[t]] + 1LL * progr[t] * fron[v[t]])) ++cnt;
  }
  int minv = cnt;
  cnt = 1;
  memset(progr, 0, sizeof(progr));
  for (int i = t - 1; i >= 1; --i) {
    if (!progr[i + 1]) {
      if ((a[v[i + 1]] - a[v[i]]) % (v[i + 1] - v[i]) == 0) {
        int r = (a[v[i + 1]] - a[v[i]]) / (v[i + 1] - v[i]);
        int ok = 0;
        if (i < t - 1) {
          if (!progr[i + 2]) ok = 1;
          if (!ok) {
            for (int j = v[i + 1]; j < v[i + 2]; ++j) {
              if (correct(a[v[i + 1]] + 1LL * (j - v[i + 1]) * r) &&
                  correct(a[v[i + 2]] -
                          1LL * (v[i + 2] - j - 1) * progr[i + 2])) {
                ok = 1;
                break;
              }
            }
          }
        } else
          ok = correct(a[v[i + 1]] + 1LL * r * fron[v[i + 1]]);
        if (ok) {
          progr[i] = r;
        } else
          progr[i] = 0, ++cnt;
      } else
        progr[i] = 0, ++cnt;
    } else {
      if ((a[v[i + 1]] - a[v[i]]) % (v[i + 1] - v[i]) == 0) {
        int r = (a[v[i + 1]] - a[v[i]]) / (v[i + 1] - v[i]);
        if (r == progr[i + 1]) {
          progr[i] = r;
        } else
          progr[i] = 0, ++cnt;
      } else
        progr[i] = 0, ++cnt;
    }
  }
  if (progr[1]) {
    if (!correct(a[v[1]] - 1LL * progr[1] * behind[v[1]])) ++cnt;
  }
  minv = min(cnt, minv);
  cout << minv;
}
