#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:64000000")
int n, m, i, j, k, l, r, u, v, w;
const int maxn = 18;
const int maxm = 110;
const int maxmask = (1 << maxn) - 1;
const int infi = 1e9 + 7 + 1e7;
const long long infl = 1e18;
const double eps = 1e-6;
const int mod = 1000;
const int alp = 26;
vector<pair<long long, long long> > ar;
int main() {
  cin >> n;
  long long d = 0, l = 0, a, b = -1;
  for (i = 0; i < n; ++i) {
    scanf("%I64d", &a);
    if (a == -1) {
      ++l;
    } else {
      if (l) {
        ar.push_back(make_pair(-1, l));
        l = 0;
      }
      ar.push_back(make_pair(a, 0));
    }
  }
  if (l) {
    ar.push_back(make_pair(-1, l));
    l = 0;
  }
  l = 1;
  pair<long long, long long> c = make_pair(0, 0);
  if (ar.size() <= 2) {
    cout << 1;
    return 0;
  }
  if (ar.size() == 3 && ar[0].first == -1 && ar[2].first == -1) {
    cout << 1;
    return 0;
  }
  if (ar[0].first == -1) {
    c.first = ar[0].second;
    a = ar[1].first;
    i = 2;
  } else {
    i = 1;
    a = ar[0].first;
  }
  int res = 0;
  for (i = i; i < ar.size(); ++i) {
    if (ar[i].first != -1) {
      if (l == 0) {
        a = ar[i].first;
        ++l;
        continue;
      }
      if (l == 1) {
        long long d;
        d = (ar[i].first - a) / (c.second + 1);
        if (d * (c.second + 1) != ar[i].first - a) {
          ++res;
          c.first = c.second = 0;
          a = ar[i].first;
          l = 1;
          continue;
        }
        if (d > 0 && c.first) {
          d *= c.first;
          d = a - d;
          if (d <= 0) {
            ++res;
            c.first = 0;
            l = 1;
            a = ar[i].first;
            c.first = c.second = 0;
            continue;
          }
          d = (ar[i].first - a) / (c.second + 1);
        }
        b = d;
        a = ar[i].first;
        c.second = 0;
        ++l;
      } else {
        long long d;
        d = b * (c.second + 1);
        if (a + d != ar[i].first) {
          ++res;
          c.second = c.first = 0;
          l = 1;
          a = ar[i].first;
          continue;
        }
        c.second = 0;
        a = ar[i].first;
        ++l;
      }
    } else {
      if (l == 1) {
        c.second += ar[i].second;
      } else {
        if (b >= 0) {
          c.second += ar[i].second;
        } else {
          long long d;
          d = b * (c.second + ar[i].second);
          d += a;
          if (d > 0) {
            c.second += ar[i].second;
          } else {
            c.second += ar[i].second;
            ++res;
            j = 0;
            while (d <= 0) {
              ++j;
              d -= b;
            }
            c.first = j;
            c.second = 0;
            l = 0;
          }
        }
      }
    }
  }
  if (c.first || l || c.second || res == 0) {
    ++res;
  }
  cout << res;
  return 0;
}
