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;
int n, aa[200005], Left[200005], Right[200005];
long long a[200005][6];
void push_back(long long x, long long *a) {
  if (x <= 0) return;
  while (a[0]) a++;
  a[0] = x;
}
void minimize(int &a, int b) {
  if (a > b) a = b;
}
bool FF[6][6][3][200005];
int F[6][6][3][200005];
int f(int u, int Just, int JustJust, int J, int JJ, int Loaded) {
  if (u == n + 1) return 1;
  if (Loaded >= 2) Loaded = 2;
  if (FF[J][JJ][Loaded][u]++) return F[J][JJ][Loaded][u];
  int Min = 0x3c3c3c3c;
  if (Loaded) minimize(Min, f(u, 0, 0, 0, 0, 0) + 1);
  for (int i = 0; int v = a[u][i]; i++)
    if (Loaded < 2 || v + JustJust == Just + Just)
      minimize(Min, f(u + 1, v, Just, i, J, Loaded + 1));
  return F[J][JJ][Loaded][u] = Min;
}
bool empty() {
  for (int i = 1; i <= n; i++)
    if (aa[i]) return false;
  return true;
}
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &aa[i]);
    if (aa[i] == -1) aa[i] = 0;
  }
  if (empty()) {
    cout << 1 << endl;
    return 0;
  }
  Left[0] = 0, Right[n + 1] = 0;
  for (int i = 1; i <= n; i++) Left[i] = (aa[i] ? i : Left[i - 1]);
  for (int i = n; i >= 1; i--) Right[i] = (aa[i] ? i : Right[i + 1]);
  for (int i = 1; i <= n; i++) {
    if (aa[i])
      push_back(aa[i], a[i]);
    else {
      if (Left[i]) push_back(aa[Left[i]], a[i]);
      if (Right[i]) push_back(aa[Right[i]], a[i]);
      if (Left[i] && Left[Left[i] - 1]) {
        int ll = Left[i], llll = Left[ll - 1];
        if ((aa[llll] - aa[ll]) % (ll - llll) == 0)
          push_back(aa[ll] + 1LL * (aa[ll] - aa[llll]) / (ll - llll) * (i - ll),
                    a[i]);
      }
      if (Right[i] && Right[Right[i] + 1]) {
        int rr = Right[i], rrrr = Right[rr + 1];
        if ((aa[rrrr] - aa[rr]) % (rr - rrrr) == 0)
          push_back(
              aa[rrrr] - 1LL * (aa[rrrr] - aa[rr]) / (rrrr - rr) * (rrrr - i),
              a[i]);
      }
      if (Left[i] && Right[i]) {
        int ll = Left[i], rr = Right[i];
        if ((aa[rr] - aa[ll]) % (rr - ll) == 0)
          push_back(aa[ll] + 1LL * (aa[rr] - aa[ll]) / (rr - ll) * (i - ll),
                    a[i]);
      }
    }
  }
  for (int i = n; i >= 1; i--) f(i, 0, 0, 0, 0, 0);
  cout << f(1, 0, 0, 0, 0, 0) << endl;
}
