import std.algorithm;
import std.array;
import std.container;
import std.conv;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.typecons;

void scan(T...)(ref T a) {
    string[] ss = readln.split;
    foreach (i, t; T) a[i] = ss[i].to!t;
}
T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

int calc(int[] a) {
    int ans = 0;
    for (int i = 2; i < a.length; i++) {
        int mi = min(a[i-2], a[i]);
        int ma = max(a[i-2], a[i]);
        if (mi < a[i-1] && a[i-1] < ma) ans++;
    }
    return ans;
}

void main() {
    readint;
    auto a = readints;
    writeln(calc(a));
}
