#include <iostream>
using namespace std;
typedef long long lll;
int n,a[1001],i,j,k,l,rj;
int main(){
	cin>>n;
	for (i=0; i<n; i++)
	cin>>a[i];
	for (i=n-2; i>=0; i--)
	{
		if (a[i+1]>a[i]) continue;
		rj+=a[i]-a[i+1]+1;
		a[i]=a[i+1]-1;
		}
	cout<<rj<<endl;
	return 0;
}