#include <bits/stdc++.h>
using namespace std;
const int N = 2 * 1e5 + 10;
const long long inf = 1e18 + 100;
const long long mod = 1e9 + 7;
void solve() {
  long long n, l, a;
  cin >> n >> l >> a;
  vector<pair<long long, long long>> mx;
  mx.push_back(make_pair(0, 0));
  for (int i = 0; i < n; i++) {
    long long l, r;
    cin >> l >> r;
    mx.push_back(make_pair(l, l + r));
  }
  mx.push_back(make_pair(l, l));
  long long ans = 0;
  for (int i = 1; i < mx.size(); i++) {
    long long tt = mx[i].first - mx[i - 1].second;
    ans += tt / a;
  }
  cout << ans;
}
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
