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 a[200005], ans, flag, kk, bb;
vector<long long> Q;
void del() {
  ans++;
  flag = 0;
  Q = vector<long long>(Q.end() - 1, Q.end());
  return;
}
int main() {
  int n, i;
  while (scanf("%d", &n) != EOF) {
    flag = 0;
    ans = 1;
    Q.clear();
    while (n--) {
      int tmp, cnt;
      scanf("%d", &tmp);
      Q.push_back(tmp);
      long long qq = kk * (Q.size() - 1) + bb;
      if (flag && (qq < 0 || tmp != -1 && tmp != qq)) {
        del();
        continue;
      }
      if (flag) continue;
      if (tmp == -1) continue;
      for (i = cnt = 0; i < Q.size(); i++)
        if (Q[i] != -1) cnt++;
      if (cnt < 2) continue;
      int aaa = -1, bbb = Q.size() - 1;
      for (i = 0; i < Q.size(); i++)
        if (Q[i] != -1) {
          aaa = i;
          break;
        }
      if ((Q[bbb] - Q[aaa]) % (bbb - aaa)) {
        del();
        continue;
      }
      long long add = (Q[bbb] - Q[aaa]) / (bbb - aaa);
      for (i = 0; i < Q.size(); i++)
        if (Q[i] != -1 && Q[i] != Q[aaa] + (i - aaa) * add ||
            Q[aaa] + (i - aaa) * add < 1)
          break;
      if (i < Q.size())
        del();
      else
        flag = 1, kk = add, bb = Q[aaa] - kk * aaa;
    }
    printf("%d\n", ans);
  }
  return 0;
}
