#include <bits/stdc++.h>
using namespace std;
const long long INF = 2000000000LL;
const long double EPS = 1e-9;
const int N = 1e5 + 7;
int a[N], d[N];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  int n, l, k;
  cin >> n >> l >> k;
  for (int i = 1; i <= n; ++i) cin >> a[i] >> d[i];
  a[0] = 0, d[0] = 0;
  a[n + 1] = l, d[n + 1] = 0;
  int res = 0;
  for (int i = 0; i <= n; ++i) {
    int gap = a[i + 1] - (a[i] + d[i]);
    res += gap / k;
  }
  cout << res << "\n";
  return 0;
}
