#include <bits/stdc++.h>
using namespace std;
void read(bool out = 0) {}
int main() {
  long long n, l, a;
  cin >> n >> l >> a;
  long long ans = 0;
  long long last = -1;
  for (int i = 0; i < n; i++) {
    long long st, en;
    cin >> st >> en;
    long long timer = st - last - 1;
    ans += timer / a;
    last = st + en - 1;
  }
  ans += (l - last - 1) / a;
  cout << ans;
}
