#include <bits/stdc++.h>
//#include <atcoder/all>
//using namespace atcoder;
using namespace std;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vvvb = vector<vvb>;
using vd = vector<double>;
using vvd = vector<vd>;
using vvvd = vector<vvd>;
#define all(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)
using pqr = priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>>;

template<class T>
bool chmax(T& p, T q) {
    if (p < q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}
template<class T>
bool chmin(T& p, T q) {
    if (p > q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

ll gcd(ll(a), ll(b)) {
    ll c = a;
    while (a % b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    return b;
}


int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    ll N;
    cin>>N;
    vll B(N);
    rep(i,N)cin>>B[i];
    ll an=0;
    ll Z=0,F=0;
    rep(i,N){
        if(B[i]==0){
            if(Z!=0){
                Z--;
                an+=i;
            }
            else{
                F++;
                an-=i;
            }
        }
        else if(B[i]>1){
            rep(j,B[i]-1){
                if(F!=0){
                    F--;
                    an+=i;
                }
                else{
                    Z++;
                    an-=i;
                }
            }
        }
    }
    cout<<an<<endl;
    
}