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 double pi = acos(-1);
const double tau = 2 * pi;
const double epsilon = 1e-9;
const int MAX_N = 200100;
int N;
long long A[MAX_N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cin >> N;
  int nSet = 0;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    if (A[i] != -1) ++nSet;
  }
  if (nSet <= 1)
    cout << "1\n";
  else {
    int lastUnset = 0;
    while (lastUnset < N && A[lastUnset] != -1) ++lastUnset;
    int first = 0;
    while (first < N && A[first] == -1) ++first;
    int second = first + 1;
    while (second < N && A[second] == -1) ++second;
    while (lastUnset < N) {
      if (first < N && second < N) {
        long long dT = (A[second] - A[first]), d = dT / (second - first);
        int start = min(first, lastUnset);
        if (dT % (second - first) == 0 && A[first] - d * (first - start) > 0) {
          A[start] = A[first] - d * (first - start);
          for (int i = start + 1; i < N; second = ++i)
            if (A[i - 1] + d > 0) {
              if (A[i] == -1)
                A[i] = A[i - 1] + d;
              else if (A[i] != A[i - 1] + d)
                break;
            } else
              break;
        } else {
          for (int i = start; i < N; second = ++i)
            if (A[i] == -1)
              A[i] = A[first];
            else if (A[i] != A[first])
              break;
        }
      } else if (first < N) {
        for (int i = lastUnset; i < N; i++) A[i] = A[first];
      } else
        break;
      while (lastUnset < N && A[lastUnset] != -1) ++lastUnset;
      first = second;
      second = first + 1;
      while (second < N && A[second] == -1) ++second;
    }
    int nSeq = 1;
    long long curDiff;
    bool isSet = false;
    for (int i = 1; i < N; i++)
      if (!isSet) {
        curDiff = A[i] - A[i - 1];
        isSet = true;
      } else if (A[i - 1] + curDiff != A[i]) {
        isSet = false;
        ++nSeq;
      }
    cout << nSeq << '\n';
  }
  return 0;
}
