#include <bits/stdc++.h>
using namespace std;
const long long MOD = (1 ? 100000000000000007 : 233333333333333333);
const int mod = (1 ? 1000000007 : 998244353);
const long long INF = 0x7fffffffffffffff;
const int inf = 0x7fffffff;
const double eps = 1e-9;
const int N = 2e5 + 500;
clock_t start, finish;
void time_in() { start = clock(); }
void time_out() {
  finish = clock();
  double tim = (double)(finish - start) / CLOCKS_PER_SEC;
  printf("Running time is %lf\n", tim);
}
inline long long mul(long long a, long long b) {
  long long s = 0;
  while (b) {
    if (b & 1) s = (s + a) % mod;
    a = (a << 1) % mod;
    b >>= 1;
  }
  return s % mod;
}
inline long long pow_mul(long long a, long long b) {
  long long s = 1;
  while (b) {
    if (b & 1) s = mul(s, a);
    a = mul(a, a);
    b >>= 1;
  }
  return s;
}
inline long long poww(long long a, long long b) {
  long long s = 1;
  while (b) {
    if (b & 1) s = (s * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return s % mod;
}
inline int read() {
  char c = getchar();
  int sgn = 1, ret = 0;
  while (c != '-' && (c < '0' || c > '9')) c = getchar();
  if (c == '-') sgn = -1, c = getchar();
  while (c >= '0' && c <= '9') ret = ret * 10 + c - 48, c = getchar();
  return ret * sgn;
}
void write(int x) {
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
inline long long phi(long long x) {
  long long ans = x;
  for (long long i = 2; i * i <= x; i++)
    if (x % i == 0) {
      ans -= ans / i;
      while (x % i == 0) x /= i;
    }
  if (x > 1) ans -= ans / x;
  return ans;
}
int t, cas;
int m, n;
long long a[3005], b[3005], dp[3005][3005];
int main() {
  while (~scanf("%d", &n)) {
    memset(dp, 0, sizeof dp);
    for (int i = 1; i <= n; i++) {
      scanf("%d", &a[i]);
      b[i] = (a[i] -= i);
    }
    sort(b + 1, b + n + 1);
    for (int i = 1; i <= n; i++) {
      long long mi = INF;
      for (int j = 1; j <= n; j++) {
        mi = min(mi, dp[i - 1][j]);
        dp[i][j] = mi + abs(a[i] - b[j]);
      }
    }
    long long ans = INF;
    for (int j = 1; j <= n; j++) ans = min(dp[n][j], ans);
    printf("%lld\n", ans);
  }
}
