#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int FFTMOD = 1007681537;
const int INF = (int)1e9;
const long long LINF = (long long)1e18;
const long double PI = acos((long double)-1);
const long double EPS = 1e-9;
inline long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
inline long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
inline int chkmin(T& a, const T& val) {
  return val < a ? a = val, 1 : 0;
}
template <class T>
inline int chkmax(T& a, const T& val) {
  return a < val ? a = val, 1 : 0;
}
inline long long isqrt(long long k) {
  long long r = sqrt(k) + 1;
  while (r * r > k) r--;
  return r;
}
inline long long icbrt(long long k) {
  long long r = cbrt(k) + 1;
  while (r * r * r > k) r--;
  return r;
}
inline void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
inline void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
inline int sign(long double x) { return x < -EPS ? -1 : x > +EPS; }
inline int sign(long double x, long double y) { return sign(x - y); }
const int maxn = 4e5 + 5;
int n;
int a[maxn];
int fen[3][maxn];
int dp[2][maxn];
void upd(int fen[], int p, int val) {
  p++;
  for (; p < maxn; p += p & -p) {
    chkmax(fen[p], val);
  }
}
int query(int fen[], int p) {
  p++;
  int res = 0;
  for (; p > 0; p -= p & -p) {
    chkmax(res, fen[p]);
  }
  return res;
}
void chemthan() {
  cin >> n;
  vector<int> dc;
  for (int i = (0); i < (n); ++i) {
    cin >> a[i], a[i] -= i;
    dc.push_back(a[i]);
    dc.push_back(a[i] + 1);
  }
  sort((dc).begin(), (dc).end()),
      (dc).erase(unique((dc).begin(), (dc).end()), (dc).end());
  int mx = 0;
  for (int i = (0); i < (n); ++i) {
    a[i] = lower_bound((dc).begin(), (dc).end(), a[i]) - dc.begin();
    dp[0][i] = query(fen[0], a[i]) + 1;
    dp[1][i] = max(query(fen[2], a[i] + 1), query(fen[1], a[i])) + 1;
    for (int j = (0); j < (2); ++j) {
      upd(fen[j], a[i], dp[j][i]);
      chkmax(mx, dp[j][i]);
    }
    if (i) {
      upd(fen[2], a[i - 1], dp[0][i - 1]);
    }
  }
  cout << max(0, n - mx - 1) << "\n";
}
int main(int argc, char* argv[]) {
  ios_base::sync_with_stdio(0), cin.tie(0);
  if (argc > 1) {
    assert(freopen(argv[1], "r", stdin));
  }
  if (argc > 2) {
    assert(freopen(argv[2], "wb", stdout));
  }
  chemthan();
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
  return 0;
}
