#include <bits/stdc++.h>
using namespace std;
int n, l, a, x, y, last, ans;
int main() {
  scanf("%d %d %d\n", &n, &l, &a);
  while (n--) {
    scanf("%d %d\n", &x, &y);
    ans += (x - last) / a;
    last = x + y;
  }
  ans += (l - last) / a;
  printf("%d\n", ans);
  return 0;
}
