#include <bits/stdc++.h>
using namespace std;

int ok( vector< int > a, int asum, int len, int &minv ){
  int mov = 0, extra = 0;
  for( int i = len; i < a.size(); ++i )
    extra += a[ i ],
    mov += a[ i ];
  int ptr = 0;
  for( int i = 1; i <= len / 2; ++i ){
    extra += a[ ptr ] - i;
    if( extra < 0 )
      return 0;
    if( a[ ptr ] - i > 0 )
      mov += abs( a[ ptr ] - i );
    ++ptr;
  }
  extra += a[ ptr ] - ( len + 1 ) / 2;
  if( extra < 0 )
    return 0;
  if( a[ ptr ] - ( len + 1 ) / 2 > 0 )
    mov += abs( a[ ptr ] - ( len + 1 ) / 2 );
  ++ptr;
  for( int i = len / 2; i >= 1; --i ){
    extra += a[ ptr ] - i;
    if( extra < 0 )
      return 0;
    if( a[ ptr ] - i > 0 )
      mov += abs( a[ ptr ] - i );
    ++ptr;
  }
  minv = min( minv, mov );
  return 1;
}

signed main(){
  int N; cin >> N;
  vector< int > A( N );
  int asum = 0;
  for( int i = 0; i < N; ++i )
    cin >> A[ i ],
    asum += A[ i ];
  int ans = 0x3f3f3f3f;
  for( int i = 1; ok( A, asum, i, ans ); i += 2 )
    ;
  cout << ans << endl;
  return 0;
}