#include <bits/stdc++.h>
using namespace std;
struct point {
  int t, l;
} A[100100];
int main() {
  int n, L, a;
  scanf("%d%d%d", &n, &L, &a);
  for (int i = 1; i <= n; i++) scanf("%d%d", &A[i].t, &A[i].l);
  int pre = 0, ans = 0;
  for (int i = 1; i <= n; i++) {
    int t = A[i].t - pre;
    ans += t / a;
    pre = A[i].t + A[i].l;
  }
  ans += (L - pre) / a;
  printf("%d\n", ans);
  return 0;
}
