#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, l, a;
  cin >> n >> l >> a;
  vector<pair<int, int> > p(n + 2);
  for (int i = 1; i <= n; i++) {
    cin >> p[i].first >> p[i].second;
    p[i].second += p[i].first;
  }
  p[n + 1].first = l;
  int answ = 0;
  for (int i = 1; i <= n + 1; i++) {
    answ += (p[i].first - p[i - 1].second) / a;
  }
  cout << answ;
}
