#include <bits/stdc++.h>
#pragma GCC diagnostic ignored "-Wunused-result"
#pragma GCC diagnostic ignored "-Wunused-function"
using namespace std;
namespace {
namespace shik {
template <class T>
void _R(T &x) {
  cin >> x;
}
void _R(int &x) { scanf("%d", &x); }
void _R(int64_t &x) { scanf("%" SCNd64, &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }
void R() {}
template <class T, class... U>
void R(T &head, U &...tail) {
  _R(head);
  R(tail...);
}
template <class T>
void _W(const T &x) {
  cout << x;
}
void _W(const int &x) { printf("%d", x); }
void _W(const int64_t &x) { printf("%" PRId64, x); }
void _W(const double &x) { printf("%.16f", x); }
void _W(const char &x) { putchar(x); }
void _W(const char *x) { printf("%s", x); }
template <class T>
void _W(const vector<T> &x) {
  for (auto i = x.begin(); i != x.end(); _W(*i++))
    if (i != x.cbegin()) putchar(' ');
}
void W() {}
template <class T, class... U>
void W(const T &head, const U &...tail) {
  _W(head);
  putchar(sizeof...(tail) ? ' ' : '\n');
  W(tail...);
}
template <class T, class F = less<T>>
void sort_uniq(vector<T> &v, F f = F()) {
  sort(begin(v), end(v), f);
  v.resize(unique(begin(v), end(v)) - begin(v));
}
template <class T>
inline T bit(T x, int i) {
  return (x >> i) & 1;
}
template <class T>
inline bool chkmax(T &a, const T &b) {
  return b > a ? a = b, true : false;
}
template <class T>
inline bool chkmin(T &a, const T &b) {
  return b < a ? a = b, true : false;
}
template <class T>
using MaxHeap = priority_queue<T>;
template <class T>
using MinHeap = priority_queue<T, vector<T>, greater<T>>;
const int N = 2e5 + 10;
int n, a[N];
int al[N], ar[N];
pair<int, int> hl[N];
int solve() {
  for (int i = (1); i <= int(n); i++) a[i] += n - i + 1;
  ;
  for (int i = (0); i <= int(n); i++) al[i] = INT_MAX;
  for (int i = (1); i <= int(n); i++) {
    int p = upper_bound(al, al + n + 1, a[i]) - al;
    hl[i] = {p, al[p]};
    al[p] = a[i];
  }
  int mx = lower_bound(al, al + n + 1, INT_MAX) - al;
  for (int i = n; i >= 1; i--) {
    int p = hl[i].first, v = hl[i].second;
    al[p] = v;
    int pr = upper_bound(ar, ar + n + 1, v - 1, greater<int>()) - ar;
    ;
    chkmax(mx, p + pr);
    if (i > 1) {
      int z = hl[i - 1].first;
      int zr = upper_bound(ar, ar + n + 1, a[i - 1] - 1, greater<int>()) - ar;
      ;
      chkmax(mx, z + 1 + zr);
    }
    int q = upper_bound(ar, ar + n + 1, a[i], greater<int>()) - ar;
    int ql = upper_bound(al, al + n + 1, a[i] + 1) - al;
    chkmax(mx, q + ql);
    ar[q] = a[i];
  }
  return max(0, n - 1 - mx);
}
void main() {
  R(n);
  for (int i = (1); i <= int(n); i++) R(a[i]);
  int ans = solve();
  W(ans);
}
}  // namespace shik
}  // namespace
int main() {
  shik::main();
  return 0;
}
