#include <bits/stdc++.h>
using namespace std;
inline long long read() {
  long long re = 0, flag = 1;
  char ch = getchar();
  while (ch > '9' || ch < '0') {
    if (ch == '-') flag = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9')
    re = (re << 1) + (re << 3) + ch - '0', ch = getchar();
  return re * flag;
}
long long n, len, x, st[100010], ed[100010];
int main() {
  n = read();
  len = read();
  x = read();
  long long i;
  for (i = 1; i <= n; i++) {
    st[i] = read();
    ed[i] = st[i] + read();
  }
  st[0] = ed[0] = 0;
  long long ans = 0;
  for (i = 0; i < n; i++) {
    ans += (st[i + 1] - ed[i]) / x;
  }
  ans += (len - ed[n]) / x;
  cout << ans;
}
