#include <algorithm>
#include <iostream>
#include <fstream>
#include <ctime>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>


using namespace std;
int a[102];
int b[102];
int main ()
{
    int n;
    cin>>n;
    int suma=0;
    for(int i=0;i<n;i++)
    {
        cin>>a[i];
        
    }
    for(int i=n-1;i>0;i--)
    {
        if(a[i]<=a[i-1]){suma+=abs(a[i-1]-a[i])+1;a[i-1]=a[i-1]-(a[i-1]-a[i])-1;}}
    cout<<suma;
}