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// Karol Kosinski 2022
#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a),_b=(b);i<_b;++i)
#define FR_(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)
#define FD_(i,b,a) for(int i=(b),_a=(a);i>=_a;--i)
#define ALL(c) (c).begin(),(c).end()
#define SIZE(c) int((c).size())
#define TIE(x...) int x;tie(x)
#define X first
#define Y second
#ifndef ENABLE_DEBUG
#define DEB(k,p,f,x...)
#else
#define DEB(k,p,f,x...) {if(k)printf("--------%4d : %s\n",__LINE__,__FUNCTION__);if(p)f(x);}
#endif
#define DEBL DEB(1,1,void,0)
#define DEBF(f,x...) DEB(1,1,f,x)
#define DEBC(p,x...) DEB(0,p,printf,x)
#define DEBUG(x...) DEB(0,1,printf,x)
using namespace std;
using LL = long long;
using ULL = unsigned long long;
using PII = pair<int, int>;
using TIII = tuple<int, int, int>;

constexpr int INFTY = 1'000'005;
constexpr int NX = 50'005;

int A[NX], B[NX], C[NX];

int solve(int n, int* S)
{
    int i = 1, r = 0;
    while ( i <= n )
    {
        if ( A[i] <= A[ i - 1 ] ) ++ S[i];
        if ( A[i] <= A[ i + 1 ] ) ++ S[i];
        i += 2;
    }
    i = 2;
    while ( i <= n )
    {
        if ( A[i] >= A[ i - 1 ] ) ++ S[i];
        if ( A[i] >= A[ i + 1 ] ) ++ S[i];
        i += 2;
    }
    FR_(j,1,n)
    {
        if ( S[j] != 2 ) continue;
        ++ r;
        DEBUG("*** %d\n", j);
        S[j] = 0;
        -- S[ j - 1 ], -- S[ j + 1 ];
    }
    FR_(j,1,n)
    {
        if ( S[j] != 1 ) continue;
        ++ r;
        DEBUG("*** %d\n", j);
        S[j] = 0;
        -- S[ j + 1 ];
    }
    DEBUG("* %d\n", r);
    return r;
}

int main()
{
    int n;
    scanf("%d", &n);
    FR_(i,1,n) scanf("%d", A + i);
    A[ n + 1 ] = ( n % 2 == 0 ) ? INFTY : -INFTY;
    A[0] = -INFTY;

    int res = solve(n, B);

    FR_(i,1,n) A[i] *= -1;
    res = min( res, solve(n, C) );

    printf("%d\n", res);
    return 0;
}