#include <iostream>
#include <cstdlib>
#include <cmath>
using namespace std;

int main()
{
    int n, raz[101], sum=0;
    scanf("%d", &n);
    for (int i=0;i<n;++i)
        scanf("%d", &raz[i]);
    for (int i=n-2;i>=0;--i)
    {
        if (raz[i]>=raz[i+1])
        {
           sum+=(raz[i]-raz[i+1]+1);
           raz[i]=raz[i+1]-1;
        }
    }
    printf("%d\n", sum);
    return 0;
}