#include <iostream>
#include <algorithm>
#include <stdio.h>

using namespace std;
int N;

struct point{
    int x;
    int y;
};

point P[1000];

double pante[1000000];


void citire(){
    cin >> N;
    for(int i=0; i<N; i++)
        cin >> P[i].x >> P[i].y;
}

double calc_panta(point point1, point point2){
    return ((double)point1.x - point2.x)/((double)point1.y - point2.y);
}


int calc(){
    int cnt = 0;
    for(int i=0; i<N-1; i++){
        if(pante[i] == pante[i+1])
            cnt++;
    }
    return cnt;
}

int main(){
    freopen("trapez.in", "r", stdin);
    freopen("trapez.out", "w", stdout);

    citire();

    for(int i=0; i<N-1; i++){
        for(int j=0; j<N-1; j++)
            if(i!=j){
                pante[i] = calc_panta(P[i], P[i+1]);
            }
    }

    sort(pante, pante+((N-1)*(N-1)-N));


     cout << calc();

    return 0;
}