#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
const int inf = 1000 * 1000 * 1000;
const int mod = 1000 * 1000 * 1000 + 7;
int n, l, a;
vector<int> g;
int main() {
  cin >> n >> l >> a;
  g.push_back(0);
  g.push_back(l);
  for (int i = 1; i <= n; i++) {
    int t, l;
    cin >> t >> l;
    g.push_back(t);
    g.push_back(t + l);
  }
  sort(g.begin(), g.end());
  int answ = 0;
  for (int i = 0; i < (int)g.size(); i += 2) {
    answ += (g[i + 1] - g[i]) / a;
  }
  cout << answ << endl;
  return 0;
}
