#include <bits/stdc++.h>
using namespace std;
int n, i;
long long int l, a, ti, li, lastbreak, ans;
int main() {
  scanf("%d", &n);
  scanf("%lld", &l);
  scanf("%lld", &a);
  i = 0;
  lastbreak = 0;
  ans = 0;
  while (i < n) {
    scanf("%lld", &ti);
    scanf("%lld", &li);
    ans += ((ti - lastbreak) / a);
    lastbreak = ti + li;
    i++;
  }
  ans += ((l - lastbreak) / a);
  printf("%lld\n", ans);
}
