#include <bits/stdc++.h>
using namespace std;
long long a[200010];
int main() {
  int n;
  while (~scanf("%lld", &n)) {
    long long ans = 0;
    for (int i = 0; i < n; ++i) scanf("%lld", &a[i]);
    for (int i = 0; i < n;) {
      ans++;
      long long j = i;
      while (j < n && a[j] == -1) j++;
      if (j == n) break;
      long long k = j + 1;
      while (k < n && a[k] == -1) k++;
      if (k == n) break;
      long long d = k - j;
      if ((a[k] - a[j]) % d) {
        i = k;
        continue;
      }
      long long s = (a[k] - a[j]) / d;
      if (s > 0 && a[j] - s * (j - i) <= 0) {
        i = k;
        continue;
      }
      long long r = k + 1;
      while (r < n) {
        long long e = a[k] + s * (r - k);
        if (a[r] != -1 && a[r] != e) break;
        if (e <= 0) break;
        r++;
      }
      i = r;
    }
    printf("%lld\n", ans);
  }
  return 0;
}
