// https://open.kattis.com/problems/walls

#include <iostream>

#include <unordered_set>

using namespace std;

typedef unordered_set<int> si;

int best = 5;

void search(si s, si::const_iterator it, int c, int acc) {

if (it == s.end()) {

if (acc == 15 && c < best) {

best = c;

}

return;

}

int val = *it;

it++;

search(s, it, c + 1, acc | val);

search(s, it, c, acc);

}

int main() {

int l, w, n, r;

cin >> l >> w >> n >> r;

si s;

for (int i = 0; i < n; i++) {

int x, y;

cin >> x >> y;

int match = 0;

double px = 0;

double py = double(-w)/2.0;

double dx = px - x;

double dy = py - y;

if (r * r >= dx * dx + dy * dy) {

match |= 1;

}

px = 0;

py = double(w)/2.0;

dx = px - x;

dy = py - y;

if (r * r >= dx * dx + dy * dy) {

match |= 2;

}

px = double(-l)/2.0;

py = 0;

dx = px - x;

dy = py - y;

if (r * r >= dx * dx + dy * dy) {

match |= 4;

}

px = double(l)/2.0;

py = 0;

dx = px - x;

dy = py - y;

if (r * r >= dx * dx + dy * dy) {

match |= 8;

}

if (match) s.insert(match);

}

search(s, s.begin(), 0, 0);

if (best == 5) {

cout << "Impossible\n";

} else {

cout << best << endl;

}

}