#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:1024000000,1024000000")
const int maxn = 100024;
const int mod = 1e9 + 7;
const double eps = 1e-8;
using namespace std;
int t[maxn], l[maxn];
int main() {
  int n, L, a;
  cin >> n >> L >> a;
  for (int i = 0; i <= n - 1; i++) {
    cin >> t[i] >> l[i];
  }
  int sum = 0, num;
  int pre = 0;
  for (int i = 0; i < n; i++) {
    num = t[i] - pre;
    sum += num / a;
    pre = t[i] + l[i];
  }
  num = L - pre;
  sum += num / a;
  cout << sum << endl;
  return 0;
}
