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

template<typename T>
void chmin(T &a,T b){
  if(a>b)a=b;
}

signed main(){
  int n;cin>>n;
  vector<int> v(n);
  for(int i=0;i<n;i++)cin>>v[i];
  int ans=1e9;
  for(int j=0;j<n;j++){
    int need=0,mv=0;
    for(int i=0;i<max(n,(j+1)*2);i++){
      int a=(i<n?v[i]:0);
      int b;
      if(i<=j)b=i+1;
      else b=j-(i-j)+1;
      if(b<0)b=0;
      if(b>a)need+=b-a;
      else mv+=a-b;
    }
    if(need>mv)continue;
    chmin(ans,mv);
  }
  cout<<ans<<endl;
}