#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  int arr[n + 10];
  for (int i = 1; i <= n; i++) scanf("%d", &arr[i]);
  int start, end;
  start = 1;
  int pos_1 = -1, pos_2 = -1;
  int cnt = 0;
  int t = 0;
  for (int i = 1; i <= n;) {
    pos_1 = -1;
    pos_2 = -1;
    start = i;
    for (int j = start; j <= n; j++) {
      if (arr[j] != -1) {
        pos_1 = j;
        break;
      }
    }
    if (pos_1 != -1) {
      for (int j = pos_1 + 1; j <= n; j++)
        if (arr[j] != -1) {
          pos_2 = j;
          break;
        }
      if (pos_2 == -1) {
        cnt++;
        break;
      }
      t++;
      if (arr[pos_1] == arr[pos_2]) {
        start = pos_2 + 1;
        for (int j = pos_2 + 1; j <= n; j++)
          if (arr[j] == -1 || arr[j] == arr[pos_1])
            start = j + 1;
          else {
            start = j;
            break;
          }
        cnt++;
      } else {
        long long val = (arr[pos_2] - arr[pos_1]);
        int d = pos_2 - pos_1;
        if (val % d == 0) {
          int yes = val / d;
          val = arr[pos_1];
          yes *= (-1);
          for (int j = pos_1 - 1; j >= start; j--) {
            val += yes;
            if (val <= 0) {
              cnt++;
              start = pos_2;
              goto End;
            }
            if (arr[j] != -1 && arr[j] != val) {
              cnt++;
              start = pos_2;
              goto End;
            }
          }
          val = arr[pos_2];
          yes *= (-1);
          start = -1;
          for (int j = pos_2 + 1; j <= n; j++) {
            val += yes;
            if (val <= 0) {
              start = j;
              break;
            }
            if (arr[j] != -1 && arr[j] != val) {
              start = j;
              break;
            }
          }
          if (start == -1) start = n + 1;
          cnt++;
        } else {
          cnt += 1;
          start = pos_2;
        }
      }
    } else {
      cnt++;
      break;
    }
  End:;
    i = start;
  }
  cout << cnt << endl;
  return 0;
}
