#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, total, a;
  cin >> n >> total >> a;
  if (n == 0) return !printf("%d\n", total / a);
  vector<pair<int, int> > v;
  for (int i = 1; i <= n; i++) {
    int t, l;
    cin >> t >> l;
    v.emplace_back(t, t + l);
  }
  sort(v.begin(), v.end());
  int ans = 0;
  ans += abs(v[n - 1].second - total) / a;
  ans += abs(v[0].first - 0) / a;
  for (int i = 0; i < n - 1; i++) {
    ans += abs(v[i + 1].first - v[i].second) / a;
  }
  cout << ans << endl;
}
