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