//  Copyright © 2017 Adán López Alatorre. All rights reserved.
//

#include <iostream>
#include <vector>
#include <string.h>
#include <deque>
#include <queue>
#include <algorithm>
#include <set>
#include <map>
#include <sstream>
#include <stack>
#include <iomanip>
#include <fstream>
#include <climits>
#include <cmath>
#define fi first
#define se second
#define FI first.first
#define SE first.second
#define TH second
#define MAXN maxN

using namespace std;

typedef long long ll;
typedef pair<int, int> ii;
typedef pair<ii, int> iii;
typedef pair<ll, ll> dd;
typedef vector<int> vi;
typedef set<ii>::iterator iter;

const int maxN = 2e5 + 3, MOD = 1e9 + 7, AL = 10;

int arr[maxN], n, act[maxN];
ll tot = 0;

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

    cin >> n;
    for(int i = 0; i < n; i++){
        cin >> arr[i];
    }

    stack<int> st;

    int cur = 0;
    for(int i = 0; i < n; i++){
        st.push(i);
        while(cur < arr[i] + 1){
            cur++;
            act[st.top()]++;
            st.pop();
        }
    }

    for(int i = 1; i < n; i++){
        act[i] += act[i - 1];
    }

    for(int i = 0; i < n; i++){
        tot += act[i] - arr[i] - 1;
    }
    cout << tot << '\n';

}