#include <bits/stdc++.h>
using namespace std;
const long long N = 1e5 + 5;
long long n, tot, a, ans = 0;
long long t[N], l[N];
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> n >> tot >> a;
  for (long long i = 1; i <= n; i++) {
    cin >> t[i] >> l[i];
    l[i] += t[i];
  }
  t[n + 1] = tot;
  for (long long i = 1; i <= n + 1; i++) ans += (t[i] - l[i - 1]) / a;
  cout << ans;
  return 0;
}
