#include <bits/stdc++.h>
using namespace std;
int n, a, L, t[100010], l[100010], res;
int main() {
  cin >> n >> L >> a;
  for (int i = 0; i < n; i++) {
    cin >> t[i] >> l[i];
  }
  t[n] = L;
  for (int i = 0; i < n; i++) {
    res += (t[i] - (t[i - 1] + l[i - 1])) / a;
  }
  res += (t[n] - (t[n - 1] + l[n - 1])) / a;
  cout << res;
  return 0;
}
