#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, U b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, U b) {
  if (a < b) a = b;
}
template <class T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T>
inline void print(T first) {
  if (first < 0) {
    putchar('-');
    return print(-first);
  }
  if (first < 10) {
    putchar('0' + first);
    return;
  }
  print(first / 10);
  putchar(first % 10 + '0');
}
template <class T, class T1>
inline void gn(T &first, T1 &second) {
  gn(first);
  gn(second);
}
template <class T, class T1, class T2>
inline void gn(T &first, T1 &second, T2 &z) {
  gn(first);
  gn(second);
  gn(z);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = 1LL * a * a % m)
    if (b & 1) ans = 1LL * ans * a % m;
  return ans;
}
inline unsigned int myrand(int n) {
  unsigned int u = rand() % n;
  int c = rand() % 10;
  u ^= u << c;
  u ^= (u << 7) & 263692864;
  u ^= (u << 15) & 402273075;
  u %= n;
  return u;
}
int dp[2][201011];
int main() {
  int n;
  cin >> n;
  int ans = 0;
  for (int i = 0; i < 201011; i++) dp[0][i] = dp[1][i] = 0x3f3f3f3f;
  dp[0][0] = -0x3f3f3f3f;
  int last = 0, pos;
  for (int i = 1, u; i <= n; i++) {
    gn(u);
    u -= i - 1;
    pos = upper_bound(dp[1], dp[1] + 201011, u) - dp[1];
    smax(ans, pos);
    dp[1][pos] = u;
    smin(dp[1][last], dp[0][last]);
    u--;
    smax(ans, last);
    last = upper_bound(dp[0], dp[0] + 201011, u) - dp[0];
    dp[0][last] = u;
  }
  cout << n - 1 - ans << endl;
  return 0;
}
