#include <iostream>


using namespace std;

int main()  {

	int n; cin >> n;
	int level[101];
	int out= 0;

	for (int i = 0; i < n; i++)
		cin >> level[i];
	
	for (int i = n-1; i >= 0; i--) 
		if (level[i-1] >= level[i]) { 
			level[i-1]--;
			out++;
			i++;
		}
	cout << out;

	return 0;
}