#include <bits/stdc++.h>
#include <atcoder/all>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace atcoder;
using namespace __gnu_pbds;
using ll=long long;
using ld=long double;
using vll=vector<ll>;
using vvll=vector<vll>;
using pll=pair<ll,ll>;

// using mint=modint;

// template<class K,class V>
// using ordered_map=tree<K,V,less<K>,rb_tree_tag,tree_order_statistics_node_update>;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

    ll N,S=0;
    cin>>N;
    vll A(N+1);
    for(int i=1;i<=N;++i)cin>>A[i],S+=A[i];

    ll ans=S;
    //1,2,...,m,...,2,1を全探索
    for(int m=1;m<=N&&m*m<=S;++m){
        ll extra=0;
        for(int i=1;i<=m;++i){
            if(i<=N&&A[i]>i)extra+=A[i]-i;
        }
        for(int i=m+1;i<=2*m-1;++i){
            int j=2*m-i;
            if(i<=N&&A[i]>j)extra+=A[i]-j;
        }
        for(int i=2*m;i<=N;++i)extra+=A[i];
        ans=min(ans,extra);
    }

    cout<<ans<<endl;

}