#include<bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
typedef long long LL;
int N;
LL A[2001];
LL dp[2001][2001];
LL m=0;
//左→右
LL lr(int x,int y){
   LL ans=A[x];
   for(int i=x+1;i<=y;i++){
       ans=max(ans+1,A[i]);
   }
   return ans;
}
//右→左
LL rl(int x,int y){
   LL ans=A[x];
   for(int i=x-1;i>=y;i--){
       ans=max(ans+1,A[i]);
   }
   return ans;
}

LL solve_right(int r,int l);
LL solve_left(int l,int r){
   if(dp[l][r]!=-1)return dp[l][r];
   if(l==r)return A[l];
   LL ans=1000000000000;
   LL calc=lr(l,r);
   ans=calc;
   if(A[l]+2*(r-l)-1<=m){
      ans=min(ans,solve_left(l+1,r));
      ans=min(ans,solve_right(r,l+1));
   }
   if(A[l]+(r-l)<=A[r])ans=min(ans,solve_right(r,l+1));
   dp[l][r]=ans;
  // cout<<"l:r="<<l<<":"<<r<<" ans"<<ans<<endl;
   return ans;
}

LL solve_right(int r,int l){
   if(dp[r][l]!=-1)return dp[r][l];
   if(l==r)return A[l];
   LL ans=1000000000000;
   LL calc=rl(r,l);
   ans=calc;
   if(A[r]+2*(r-l)-1<=m){
      ans=min(ans,solve_right(r-1,l));
      ans=min(ans,solve_left(l,r-1));
   }
   if(A[r]+(r-l)<=A[r])ans=min(ans,solve_right(l,r-1));
   dp[r][l]=ans;
   return ans;
}

int main(){
        REP(i,2001){
            REP(j,2001){
                dp[i][j]=-1;
            }
        }
	cin>>N;
	REP(i,N){
	    cin>>A[i];
	    m=max(m,A[i]);
	}
        cout<<min(solve_left(0,N-1),solve_right(N-1,0))<<endl;
	return(0);
}