#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0;i < n;i++)
#define VSORT(v) sort(v.begin(), v.end())
#define VRSORT(v) sort(v.rbegin(), v.rend())
#define ll long long
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;
//typedef vector<unsigned int>vec;
typedef vector<ll>vec;
typedef vector<vec> mat;
typedef vector<vector<int>> Graph;

const int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
const int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};
const int INF = 1000000000;
const ll LINF = 1000000000000000000;//1e18
const ll  MOD = 1000000007;
//const ll MOD = 998244353;
const double PI = acos(-1.0);
const double EPS = 1e-10;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline void add(T &a, T b){a = ((a+b) % MOD + MOD) % MOD;};


int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N;
    cin >> N;
    vector<int> B(N);
    set<int> st;
    st.insert(-1);
    st.insert(N+1);

    REP(i,N){
        cin >> B[i];
        if(B[i] > 1) st.insert(i);
    }
    int ans = 0;
    REP(i,N){
        if(B[i] >= 1) continue;
        auto itr = st.lower_bound(i);
        auto itr2 = itr;
        itr2--;
        //cout << *itr << endl;
        if(*itr2 == -1){
            B[*itr]--;
            ans += abs(*itr - i);
            if(B[*itr] <= 1) st.erase(*itr);
        }
        else if(*itr == N+1){
            //cout << i << endl;
            B[*itr2]--;
            ans += abs(*itr2 - i);
            if(B[*itr2] <= 1) st.erase(*itr2);
        }
        else{
            if(*itr2 - i <= *itr - i){
                B[*itr2]--;
                ans += abs(*itr2 - i);
                if(B[*itr2] <= 1) st.erase(*itr2);
            }
            else{
                B[*itr]--;
                ans += abs(*itr - i);
                if(B[*itr] <= 1) st.erase(*itr);
            }
        }
    }
    cout << ans << endl;
}