#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
int n, L, a, t[N], l[N];
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> L >> a;
  if (n == 0) {
    cout << L / a;
    return 0;
  }
  for (int i = 0; i < n; i++) cin >> t[i] >> l[i];
  int ans = 0;
  ans += t[0] / a;
  t[n] = L;
  for (int i = 0; i < n; i++) {
    int tmp = t[i + 1] - t[i] - l[i];
    ans += tmp / a;
  }
  cout << ans;
  return 0;
}
