#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
#define ll long long
#define ld long double
#define INF 1000000000000000000
typedef pair<ll, ll> pll;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N;
    cin >> N;
    stack<pll> st;
    ll ans = 0;
    rep(i, N) {
        ll a;
        cin >> a;
        if (a == 1)
            continue;
        if (st.empty()) {
            st.push({a, i});
        } else if (a == 0) {
            int tar = st.top().first;
            if (tar == 0) {
                st.push({a, i});
            } else {
                st.top().first--;
                ans += abs(st.top().second - i);
                if (st.top().first == 0)
                    st.pop();
            }
        } else {
            while (1) {
                if (st.empty()) {
                    st.push({a, i});
                    break;
                }
                int tar = st.top().first;
                if (tar != 0) {
                    st.push({a, i});
                    break;
                } else {
                    a--;
                    ans += abs(st.top().second - i);
                    st.pop();
                    if (a == 0)
                        break;
                }
            }
        }
    }

    cout << ans << endl;
}