#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <vector>
#define pb push_back

using namespace std;

vector<int>lvl;

int main(){
    int n, a; cin >> n;
    for(int i=0; i<n; i++){
        cin >> a;
        lvl.pb(a);
    }
    int count=0;
    for(int i=n-2; i>=0; i--){
        if(lvl[i]>=lvl[i+1]){
            count+=lvl[i]-lvl[i+1]+1;
            lvl[i]=lvl[i+1]-1;
        }
    }
    cout << count << endl;


    return 0;
}