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#include <stdio.h>
#include <time.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <map> 
#include <string>
#include <vector>  
#include <iostream> 
#include <sstream> 
#include <queue>
#include <algorithm>
#include <assert.h>


using namespace std;

#define ll long long
#define PB 		push_back
#define FOR(a,start,end) 	for(int a=int(start); a<int(end); a++)
#define INF 		INT_MAX
#define SORT(a) 	sort(a.begin(),a.end()) 
#define CL(a,x) 		memset(a,x,sizeof(a))
#define REP(a,x)	for(int a=0;a<x;a++)
#define REP1(a,x)	for(int a=1;a<=x;a++)
#define MP 		make_pair



typedef vector<ll>     vi;
typedef pair<ll, ll>     pii;
typedef vector<string> vs;
typedef vector<pii> vii;
typedef vector<vector<ll> > vvi;
typedef vector<vector<string> > vvs;
typedef vector<pair<string, string> > vss;
typedef pair<string, string> pss;
typedef pair<ll, pii> ppii;
typedef vector<ppii> vppii;
typedef vector<vector<pii> > vvii;
typedef vector<vvi> vvvi;


ll  n, k,k1,k2;
string wa;
vi vk1,vk2;

int main() {
	//freopen("c:\\wojtek\\uva\\pa\\debug\\orn0.in", "rt", stdin);

	//while (1) {

	cin >> n;
	k1 = 0;
	k2 = 0;
	vk1.assign(n, 0);
	vk2.assign(n, 0);
	
	for (int i = 0; i < n; i++) {
		cin >> vk1[i];
		vk2[i]=vk1[i];
	}
	
	for (int i = 1; i < n-2; i+=2) {
		if (vk1[i] > vk1[i - 1]) {
			if (vk1[i] <= vk1[i + 1]) {
				vk1[i + 1] = min(vk1[i] - 1, vk1[i + 2] - 1);
				k1++;
			}
		}
		else {
			vk1[i] = max(vk1[i - 1] + 1, vk1[i + 1] + 1);
			k1++;
		}

		if (vk2[i] < vk2[i - 1]) {
			if (vk2[i] >= vk2[i + 1]) {
				vk2[i + 1] = max(vk2[i] + 1, vk2[i + 2] + 1);
				k2++;
			}
		}
		else {
			vk2[i] = min(vk2[i - 1] - 1, vk2[i + 1] - 1);
			k2++;
		}
		
	}
	if (n % 2) {
		if ((vk1[n - 2] <= vk1[n - 3]) || (vk1[n - 2] <= vk1[n - 1]))
			k1++;
		if ((vk2[n - 2] >= vk2[n - 3]) || (vk2[n - 2] >= vk2[n - 1]))
			k2++;
	}
	else {
		if ((n % 2) == 0) {
			if (vk1[n - 1] <= vk1[n - 2])
				k1++;

			if (vk2[n - 1] >= vk2[n - 2])
				k2++;
		}
	}
	cout << min(k1,k2) << endl;
	// }

	return 0;

}