#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l, a;
  cin >> n >> l >> a;
  int ans = 0, r = 0;
  for (int i = 0; i < n; i++) {
    int t, d;
    cin >> t >> d;
    ans += (t - r) / a;
    r = t + d;
  }
  ans += (l - r) / a;
  cout << ans << endl;
}
