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