#include <stdio.h>
#include <set>

int x[200010];
std::set< std::pair<int,int> > S;
int main()
{
	int ans = 0;
	int a;
	scanf("%d",&a);
	for(int i=1;i<=a;i++) scanf("%d",&x[i]);
	for(int i=2;i<=a-1;i++) S.insert(std::make_pair(x[i],i));
	while(!S.empty())
	{
		std::set< std::pair<int,int> > ::iterator it = S.end();
		it--;
		int ind = (it->second);
		S.erase(it);
		if(x[ind-1]<x[ind] && x[ind]>x[ind+1])
		{
			ans++;
			x[ind] = (x[ind-1]<x[ind+1]?x[ind-1]:x[ind+1])-1;
			S.insert(std::make_pair(x[ind],ind));
		}
	}
	printf("%d",ans);
}