#include <bits/stdc++.h>
using namespace std;
const double PI = acos(0) * 2;
const double EPS = 1e-8;
const long long MOD = 1e9 + 7;
const int MAXN = 1e5 + 5;
const int oo = 1e9;
const double foo = 1e30;
template <class T>
int getbit(T s, int i) {
  return (s >> i) & 1;
}
template <class T>
T onbit(T s, int i) {
  return s | (T(1) << i);
}
template <class T>
T offbit(T s, int i) {
  return s & (~(T(1) << i));
}
template <class T>
int cntbit(T s) {
  return __builtin_popcounll(s);
}
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; }
int n, a[MAXN];
vector<int> v;
long long f[MAXN];
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    a[i] -= i;
    v.push_back(a[i]);
  }
  sort(v.begin(), v.end());
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      f[j] += (long long)abs(a[i] - v[j]);
      if (j) f[j] = min(f[j], f[j - 1]);
    }
  }
  cout << f[n - 1] << endl;
}
