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 eps = 1e-9;
const double pi = 2 * acos((double)0);
int val[200005], nn;
int NextSeqStart(int x) {
  int significantVal[3], nSignificant = 0;
  int idx = x;
  while (idx < nn && nSignificant < 2) {
    if (val[idx] > 0) {
      significantVal[nSignificant] = idx;
      nSignificant++;
    }
    idx++;
  }
  if (nSignificant < 2) return nn;
  int firstIdx = significantVal[0], secondIdx = significantVal[1];
  int firstVal = val[firstIdx], secondVal = val[secondIdx];
  if ((secondVal - firstVal) % (secondIdx - firstIdx) != 0) return secondIdx;
  int diff = (secondVal - firstVal) / (secondIdx - firstIdx);
  idx = secondIdx;
  int currentVal = secondVal;
  while (idx >= x) {
    if (currentVal <= 0) return secondIdx;
    currentVal -= diff;
    idx--;
  }
  idx = secondIdx;
  currentVal = secondVal;
  while (idx < nn) {
    if (currentVal <= 0) break;
    if (val[idx] != -1 && val[idx] != currentVal) break;
    idx++;
    currentVal += diff;
  }
  return idx;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin >> nn;
  for (long long i = (0); i < (nn); i++) cin >> val[i];
  int start = 0;
  int nseq = 0;
  while (start < nn) {
    nseq++;
    start = NextSeqStart(start);
  }
  cout << nseq << endl;
  return 0;
}
