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;
const int INF = 1e9 + 10;
const long long int INFLL = 1e18 + 10;
const long double EPS = 1e-8;
const long double EPSLD = 1e-14;
const long long int MOD = 1e9 + 7;
template <class T>
T &chmin(T &a, const T &b) {
  return a = min(a, b);
}
template <class T>
T &chmax(T &a, const T &b) {
  return a = max(a, b);
}
const int MAX_N = 200010;
int n;
long long int a[MAX_N];
int ans = 0;
int main() {
  scanf("%d", &n);
  for (int i = (0); i < (int)(n); i++) scanf("%lld", &a[i]);
  long long int d = INFLL;
  int p = -1;
  int skipped = INF;
  for (int i = (0); i < (int)(n); i++) {
    if (a[i] == -1) {
      if (d != INFLL) {
        if (a[i - 1] + d > 0)
          a[i] = a[i - 1] + d;
        else
          d = INFLL, p = -1, skipped = i;
      } else {
        chmin(skipped, i);
      }
    } else {
      if (d != INFLL) {
        if (a[i] != a[i - 1] + d) {
          p = i;
          d = INFLL;
        }
      } else if (d == INFLL && p >= 0) {
        if ((a[i] - a[p]) % (i - p) == 0) {
          d = (a[i] - a[p]) / (i - p);
          if (skipped == INF || d * (skipped - p) + a[p] > 0) {
            for (int j = (skipped); j < (int)(i); j++)
              a[j] = d * (j - p) + a[p];
          } else {
            for (int j = (skipped); j < (int)(i); j++) a[j] = a[p];
            p = i;
            d = INFLL;
          }
        } else {
          for (int j = (skipped); j < (int)(i); j++) a[j] = a[p];
          p = i;
        }
        skipped = INF;
      } else if (d == INFLL && p == -1) {
        p = i;
      }
    }
  }
  if (d == INFLL && p >= 0 && skipped != INF) {
    for (int j = (skipped); j < (int)(n); j++) a[j] = a[p];
  }
  d = INFLL;
  for (int i = (1); i < (int)(n); i++) {
    if (d != INFLL) {
      if (a[i] != a[i - 1] + d) {
        d = INFLL;
        ans++;
      }
    } else {
      d = a[i] - a[i - 1];
    }
  }
  printf("%d\n", ans + 1);
  return 0;
}
