#include <bits/stdc++.h>
using namespace std;
signed main() {
  long long n, l, a;
  long long c = 0;
  cin >> n >> l >> a;
  long long x1, y1, z1, x2, y2, z2;
  if (n >= 1) {
    cin >> x1 >> y1;
    z1 = x1 + y1;
    c += (x1 / a);
    for (long long i = 1; i < n; ++i) {
      cin >> x2 >> y2;
      z2 = x2 + y2;
      c += (x2 - z1) / a;
      z1 = z2;
    }
    c += (l - z1) / a;
    cout << c;
  } else
    cout << l / a;
  return 0;
}
