#include <bits/stdc++.h>
using namespace std;
int n, l, a;
int wyn = 0;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> l >> a;
  int ost = 0;
  for (int i = 0; i < (n); ++i) {
    int t1, t2;
    cin >> t1 >> t2;
    wyn += (t1 - ost) / a;
    ost = t1 + t2;
  }
  wyn += (l - ost) / a;
  cout << wyn;
  return 0;
}
