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