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 ans = 0, j, n;
int a[20001];
void set(int m) {
  int i, flag = 0, nn = 0, i2;
  int cha, qian;
  if (m == n - 1) {
    j = n;
    return;
  } else if (a[m] != -1) {
    for (i = m + 1; i < n; i++) {
      if (a[i] != -1) {
        flag = 1;
        break;
      }
    }
    if (flag == 0) {
      j = n;
      return;
    }
    if ((a[i] - a[m]) % (i - m) != 0) {
      j = i;
      return;
    }
    cha = (a[i] - a[m]) / (i - m);
    if (i == n - 1) {
      j = n;
      return;
    } else {
      qian = a[i];
      for (j = i + 1; j < n; j++) {
        if (qian + cha <= 0)
          return;
        else {
          if (qian + cha != a[j] && a[j] != -1)
            return;
          else
            qian = qian + cha;
        }
      }
    }
  } else if (a[m] == -1) {
    for (i = m + 1; i < n; i++) {
      if (a[i] != -1) {
        if (nn == 0) {
          i2 = i;
        }
        nn++;
        if (nn == 2) break;
      }
    }
    if (nn == 0) {
      j = n;
      return;
    } else if (nn == 1) {
      if (i2 - m < a[i2]) {
        j = n;
        return;
      } else if (n - 1 - i2 < a[i2]) {
        j = n;
        return;
      } else {
        ans++;
        j = n;
        return;
      }
    } else if (nn == 2) {
      if ((a[i] - a[i2]) % (i - i2) != 0) {
        j = i;
        if (i2 - m < a[i2] || n - 1 - i2 < a[i2]) {
          return;
        } else {
          j = i2 + 1;
          return;
        }
      } else {
        cha = (a[i] - a[i2]) / (i - i2);
        if (a[i] - (i - m) * cha < 0) {
          ans++;
        }
        if (i == n - 1) {
          j = n;
          return;
        } else {
          qian = a[i];
          for (j = i + 1; j < n; j++) {
            if (qian + cha <= 0)
              return;
            else {
              if (qian + cha != a[j] && a[j] != -1)
                return;
              else
                qian = qian + cha;
            }
          }
        }
      }
    }
  }
}
int main(void) {
  int i;
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  j = 0;
  while (j < n) {
    ans++;
    set(j);
  }
  cout << ans << endl;
}
