#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e18;
const int maxn = 3000 + 10;
int n, m, A[maxn], B[maxn];
long long f[maxn][maxn];
void init() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &A[i]);
    A[i] -= i;
    B[i] = A[i];
  }
}
void solve() {
  sort(B + 1, B + n + 1);
  m = 1;
  for (int i = 2; i <= n; ++i)
    if (B[m] != B[i]) B[++m] = B[i];
  for (int i = 1; i <= n; ++i)
    for (int j = 0; j <= m; ++j) f[i][j] = inf;
  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= m; ++j)
      f[i][j] = min(f[i][j - 1], f[i - 1][j] + abs(A[i] - B[j]));
  long long ans = inf;
  for (int i = 1; i <= m; ++i) ans = min(ans, f[n][i]);
  printf("%I64d\n", ans);
}
int main() {
  init();
  solve();
  return 0;
}
