#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l, a;
  cin >> n >> l >> a;
  int prev = 0;
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    int ti, li;
    cin >> ti >> li;
    ans += (ti - prev) / a;
    prev = ti + li;
  }
  ans += (l - prev) / a;
  cout << ans << endl;
}
