#include <bits/stdc++.h>
using namespace std;
const int P = 1e9 + 7, INF = 0x3f3f3f3f;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long qpow(long long a, long long n) {
  long long r = 1 % P;
  for (a %= P; n; a = a * a % P, n >>= 1)
    if (n & 1) r = r * a % P;
  return r;
}
long long inv(long long first) {
  return first <= 1 ? 1 : inv(P % first) * (P - P / first) % P;
}
inline int rd() {
  int first = 0;
  char p = getchar();
  while (p < '0' || p > '9') p = getchar();
  while (p >= '0' && p <= '9') first = first * 10 + p - '0', p = getchar();
  return first;
}
const int N = 3e3 + 10;
int n, a[N], b[N];
long long dp[N][N];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) scanf("%d", a + i), a[i] -= i, b[i] = a[i];
  sort(b + 1, b + 1 + n);
  for (int i = 1; i <= n; ++i) {
    for (int j = 1; j <= n; ++j) dp[i][j] = dp[i - 1][j] + abs(a[i] - b[j]);
    for (int j = 1; j <= n; ++j)
      dp[i][j] = min(dp[i][j], dp[i - 1][j] + abs(a[i] - b[j]));
    for (int j = 2; j <= n; ++j) dp[i][j] = min(dp[i][j], dp[i][j - 1]);
  }
  printf("%lld\n", dp[n][n]);
}
