#include <bits/stdc++.h>
using namespace std;
pair<int, int> arr[100005];
int n, l, a;
int main() {
  cin >> n >> l >> a;
  arr[0].first = 0;
  arr[0].second = 0;
  for (int i = 1; i <= n; i++) {
    cin >> arr[i].first >> arr[i].second;
  }
  sort(arr, arr + n + 1);
  int cnt = 0;
  for (int i = 0; i <= n; i++) {
    int nyak;
    if (i == n) {
      nyak = l;
    } else {
      nyak = arr[i + 1].first;
    }
    nyak = nyak - (arr[i].first + arr[i].second);
    cnt = cnt + nyak / a;
  }
  cout << cnt << "\n";
}
