#include <iostream>
#include <cstdio>
#include <cstring>
#include <vector>
#include <set>
#include <algorithm>
#include <cmath>

using namespace std;

int n, sol, a[105];

int main(){
    a[0]=-1337;

    scanf("%d", &n);
    for (int i=1; i<=n; i++)
        scanf("%d", &a[i]);
    for (int i=n; i; i--)
        if (a[i]<=a[i-1]){
            sol+=a[i-1]-a[i]+1;
            a[i-1]=a[i]-1;
        }
    printf("%d\n", sol);
	return 0;
}