#include <bits/stdc++.h>
using namespace std;
template <typename T>
ostream& operator<<(ostream& out, vector<T> v) {
  for (T t : v) out << t << ' ';
  return out;
}
template <typename T>
ostream& operator<<(ostream& out, set<T> v) {
  for (T t : v) out << t << ' ';
  return out;
}
template <typename T>
ostream& operator<<(ostream& out, pair<T, T> v) {
  out << v.first << ' ' << v.second;
  return out;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n, l, a;
  cin >> n >> l >> a;
  int prev = 0;
  long long res = 0;
  for (int i = 0; i < n; ++i) {
    int t, len;
    cin >> t >> len;
    res += (t - prev) / a;
    prev = t + len;
  }
  res += (l - prev) / a;
  cout << res;
  return 0;
}
