#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, l, a, ans = 0;
  cin >> n >> l >> a;
  vector<pair<long long, long long> > us(n);
  for (size_t i = 0; i < n; i++) {
    cin >> us[i].first;
    ;
    cin >> us[i].second;
    ;
  }
  if (us.size() == 0) {
    cout << l / a;
    return 0;
  }
  for (int i = us.size() - 1; i >= 0; i--) {
    ans += (l - (us[i].second + us[i].first)) / a;
    l -= (l - us[i].first);
  }
  ans += l / a;
  cout << ans;
  return 0;
}
