#include<iostream>
#include<sstream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<numeric>
#include<functional>
#include<complex>

using namespace std;
#define BET(a,b,c) ((a)<=(b)&&(b)<(c))
#define FOR(i,n) for(int i=0,i##_end=(int(n));i<i##_end;i++)
#define SZ(x) (int)(x.size())
#define ALL(x) (x).begin(),(x).end()
#define MP make_pair
#define FOR_EACH(it,v) for(__typeof(v.begin()) it=v.begin(),it_end=v.end() ; it != it_end ; it++)
typedef vector<int> VI;
typedef vector<VI> VVI;

int main()
{
    int N;
    cin>>N;
    VI A(N);
    FOR(i,N) cin>>A[i];
    int sum = 0 ;
    FOR(i,N) sum += A[i];
    int L = 1;
    while((L+1)*(L+1)<=sum) L++;
    int up = 0, down = 0 ;
    FOR(i,2*L-1){
        int x = i < SZ(A) ? A[i] : 0;
        int y = min(i+1, 2 * L - 1 - i);
        //cout<<i<<" "<<x<<" "<<y<<endl;
        if(x < y) up += y - x;
        else      down += x - y; 
    }
    for(int j=2*L-1;j<SZ(A);j++) down += A[j];
    cout<<max(up, down)<<endl;
    return 0;
}