#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 10;
const int MOD = 1e9 + 7;
int n, l, a, ans;
pair<int, int> c[maxn];
void solve() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  cin >> n >> l >> a;
  int x = 0;
  for (int i = 1; i <= n; i++) {
    cin >> c[i].first >> c[i].second;
    ans += (c[i].first - x) / a;
    x = c[i].first + c[i].second;
  }
  ans += (l - x) / a;
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
