#include <bits/stdc++.h>
using namespace std;
long long n, L, a;
long long s[200000], ss[200000];
int main() {
  cin >> n >> L >> a;
  long long ans = 0;
  long long t = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d %d", &s[i], &ss[i]);
    ans += (s[i] - t) / a;
    t = s[i] + ss[i];
  }
  ans += (L - t) / a;
  cout << ans << endl;
  return 0;
}
