#include <bits/stdc++.h>
using namespace std;
pair<long long, long long> v[1123456];
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  long long n, l, a;
  cin >> n >> l >> a;
  for (long long i = 0; i < n; i++) cin >> v[i].first >> v[i].second;
  for (long long i = 0; i < n; i++) v[i].second += v[i].first;
  sort(v, v + n);
  long long resp = 0;
  for (long long i = 0; i < n - 1; i++)
    resp += ((v[i + 1].first - v[i].second) / a);
  if (n >= 1) resp += v[0].first / a;
  resp += (l - v[n - 1].second) / a;
  cout << resp << endl;
  return 0;
}
