#include<bits/stdc++.h>
#define int long long
#define double long double
//#define endl '\n'
#define pll pair<int,int>
using namespace std;

const int N = 300 + 10, M = 2e5 + 10;

int x[N], y[N];

int cha (pll a, pll b) {
    return a.first * b.second - a.second * b.first;
}

void solve () {
    int n;
    cin >> n;
    for (int i = 0; i < n; i ++) {
        cin >> x[i] >> y[i];
    }

    int ans = 0;
    for (int i = 0, j = 1; i < n; i ++) {
        while (y[i] == y[j]) j = (j + 1) % n;

        int pre = (i - 1 + n) % n;
        if (y[pre] > y[i] && y[j] > y[i]) {
            pll a = {x[i] - x[pre], y[i] - y[pre]}, b = {x[j] - x[i], y[j] - y[i]};
            if (cha(a, b) > 0) {
                //cout << i + 1 << " " << j + 1 << endl;
                ans ++;
            }
        }
    }

    cout << max(1ll, ans) << endl;
}

signed main () {
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
    int t = 1;

    //cin >> t;
    while (t --) {
        solve ();
    }
    return 0;
}
/*

*/