#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int n, m, a, t[100100], l[100100], ans;
int main() {
  n = read(), m = read(), a = read();
  for (int i = 1; i <= n; i++) t[i] = read(), l[i] = read();
  t[++n] = m;
  for (int i = 0; i < n; i++) ans += max((t[i + 1] - t[i] - l[i]) / a, 0);
  printf("%d", ans);
}
