#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

int main(){
	int n,rj=0;cin>>n;
	int a[128];
	for (int i=0; i<n; ++i) cin>>a[i];
	for (int i=n-2;i>-1; --i){
	    if (a[i]>=a[i+1]) {rj+=a[i]-a[i+1]+1; a[i]-=(a[i]-a[i+1])+1;}
	}
	/*for (int i=0; i<n; ++i) {
	    cout<<a[i]<<" ";}*/
	cout<<rj;
	return 0;
}