#include <iostream>
#include <cstdlib>
#include <cstdio>


using namespace std;

int n, p[100], br;
int main ()
{
    scanf("%d", &n);
    for (int i=0; i<n; i++) scanf("%d", &p[i]);
    for (int i=n-1; i>0; i--)
    {
        while (p[i-1]>=p[i])
        {
            p[i-1]=p[i-1]-1;
            br=br+1;
        }
    }
    printf("%d", br);
return 0;
}