#include <fstream>
#include <cmath>
#include <algorithm>
using namespace std;

struct Point {
    int x, y;
};

const int INF = INT_MAX;
double eps;

double dist(Point P1, Point P2) {
    return ( sqrt( ( (P2.x - P1.x) * (P2.x - P1.x) ) + ( (P2.y - P1.y) * (P2.y - P1.y) ) ) );
}

bool SamePoint(Point P1, Point P2) {
    if ( ( fabs(P1.x - P2.x) < eps ) && ( fabs(P1.y - P2.y) < eps ) ) {
        return 1;
    }
    return 0;
}

Point median(Point P1, Point P2) {
    Point M;
    M.x = (P1.x + P2.x) / 2;
    M.y = (P1.y + P2.y) / 2;
    return M;
}

double slope(Point P1, Point P2) {
    if (fabs(P2.x - P1.x) < eps)
        return INF;
    else
        return ((P2.y - P1.y) / (P2.x - P1.x));
}

ifstream I("trapez.in");
ofstream O("trapez.out");

const int GMAN = 10005;
double pante[GMAN];
Point toyota[GMAN];



int main(){
    int n, Ebony = 0, Ivory = 0, c = 0, r = 0;
    I >> n;
    for (int i = 0; i < n; i++) {
        I >> toyota[i].x >> toyota[i].y;
    }

    while (true) {
        if (Ivory = n) {
            Ebony++;
            Ivory = Ebony + 1;
            if (Ebony >= n) break;
        }
        else {
            Ivory++;
        }

        pante[c] = slope(toyota[Ebony], toyota[Ivory]);
        c++;

    }

    //sort(pante[0], pante[c]);

    for (int i = 0; i < c; i++) {
        if (!pante[i]) r++;
    }
    O << r;
    return 0;
}