#include <bits/stdc++.h>
using namespace std;
using PP = pair<long, long>;
long n, d, a;
set<PP> st;
int main() {
  cin >> n >> d >> a;
  for (int i = 0; i < n; ++i) {
    int x, h;
    cin >> x >> h;
    h = (h + a - 1) / a;
    st.insert(PP(x, -h));
  }
  
  long sum = 0;
  long cnt = 0;
  for (auto it = st.begin(); it != st.end(); ++it) {
    PP p = *it;
    long x = p.first;
    long k = p.second;
    if (k < 0) {
      long h = -k;
      long at = h - cnt;
      if (at > 0) {
        sum += at;
        cnt += at;
        st.insert(PP(x + 2 * d, at));
      }
    } else {
      int at = k;
      cnt -= at;
    }
  }
  cout << sum << endl;
}