#include<bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
typedef long long ll;
int cnt(int h, int a) {
  return h % a ? h / a + 1 : h / a;
}
int main() {
  ll n, d, a, sum=0,x=0;
  int h, k=0, l=0;
  cin >> n >> d >> a;
  vector<pair<ll, int>> c(n),v(n);
  rep(i,n)
    cin >> c[i].first >> c[i].second;
  sort(c.begin(), c.end());
  rep(i,n){
    for (int j=k; j<l;++j)
      if(v[j].first<c[i].first){
        k++;
        x-=v[j].second;
      }else break;
    c[i].second-=x;
    if (c[i].second > 0) {
      h = cnt(c[i].second, a);
      sum += h;
      h *= a;
      x+=h;
      v[l].first = c[i].first+2*d;
      v[l].second = h;
      l++;
    }
  }
  cout << sum;
}
