#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int N, L, T;
  cin >> N >> L >> T;
  static int S[100005], len[100005];
  for (int i = 1; i <= N; i++) cin >> S[i] >> len[i];
  if (N == 0) {
    cout << L / T;
  } else {
    int ans = S[1] / T + (L - S[N] - len[N]) / T;
    for (int i = 1; i < N; i++) ans += (S[i + 1] - S[i] - len[i]) / T;
    cout << ans;
  }
}
