#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int TESTS = 1;
  while (TESTS--) {
    long long int n, l, a;
    cin >> n >> l >> a;
    vector<long long int> v;
    v.push_back(0);
    v.push_back(l);
    while (n--) {
      long long int t, li;
      cin >> t >> li;
      v.push_back(t);
      v.push_back(t + li);
    }
    sort((v).begin(), (v).end());
    long long int ans = 0;
    for (long long int i = 1; i < v.size(); i += 2) {
      long long int tmp = (v[i] - v[i - 1]) / a;
      ans += tmp;
    }
    cout << ans;
  }
  return 0;
}
