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