#include <stdio.h>
#define N 1000000
unsigned int X = 12345;
int rand_() {
return (X *= 3) >> 1;
}
long long cross(long long x1, long long y1, long long x2, long long y2) {
return x1 * y2 - x2 * y1;
}
long long pp[N], qq[N];
void sort(int *ii, int l, int r) {
while (l < r) {
int i = l, j = l, k = r, i_ = ii[l + rand_() % (r - l)], tmp;
while (j < k) {
long long c = cross(pp[ii[j]], qq[ii[j]], pp[i_], qq[i_]);
if (c == 0)
j++;
else if (c < 0) {
tmp = ii[i], ii[i] = ii[j], ii[j] = tmp;
i++, j++;
} else {
k--;
tmp = ii[j], ii[j] = ii[k], ii[k] = tmp;
}
}
sort(ii, l, i);
l = k;
}
}
int main() {
static int xx[N], yy[N], ii[N];
int n, y, i, p, q, k;
scanf("%d%d", &n, &y);
for (i = 0; i < n; i++)
scanf("%d%d", &xx[i], &yy[i]);
for (i = 2; i < n - 1; i += 2) {
pp[i] = cross(xx[i - 1], yy[i - 1] - y, xx[i] - xx[i - 1], yy[i] - yy[i - 1]);
qq[i] = cross(1, 0, xx[i] - xx[i - 1], yy[i] - yy[i - 1]);
if (qq[i] < 0)
pp[i] = -pp[i], qq[i] = -qq[i];
pp[i + 1] = cross(xx[i + 1], yy[i + 1] - y, xx[i] - xx[i + 1], yy[i] - yy[i + 1]);
qq[i + 1] = cross(1, 0, xx[i] - xx[i + 1], yy[i] - yy[i + 1]);
if (qq[i + 1] < 0)
pp[i + 1] = -pp[i + 1], qq[i + 1] = -qq[i + 1];
ii[i / 2 - 1] = i + 1;
}
sort(ii, 0, n / 2 - 1);
p = -1, q = 0, k = 0;
for (i = 0; i < n / 2 - 1; i++)
if (cross(p, q, pp[ii[i] - 1], qq[ii[i] - 1]) < 0)
p = pp[ii[i]], q = qq[ii[i]], k++;
printf("%d\n", k);
return 0;
}
