#include <bits/stdc++.h>
const int inf = 0x3f3f3f3f, Inf = 0x7fffffff;
const long long INF = 0x7fffffffffffffff;
const double eps = 1e-10;
template <typename _Tp>
_Tp gcd(const _Tp &a, const _Tp &b) {
  return (!b) ? a : gcd(b, a % b);
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) _Tp abs(const _Tp &a) {
  return a >= 0 ? a : -a;
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) _Tp max(const _Tp &a, const _Tp &b) {
  return a < b ? b : a;
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) _Tp min(const _Tp &a, const _Tp &b) {
  return a < b ? a : b;
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) void chmax(_Tp &a, const _Tp &b) {
  (a < b) && (a = b);
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) void chmin(_Tp &a, const _Tp &b) {
  (b < a) && (a = b);
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) bool _cmp(const _Tp &a,
                                                    const _Tp &b) {
  return abs(a - b) <= eps;
}
template <typename _Tp>
__inline__ __attribute__((always_inline)) void read(_Tp &x) {
  register char ch(getchar());
  bool f(false);
  while (ch < 48 || ch > 57) f |= ch == 45, ch = getchar();
  x = ch & 15, ch = getchar();
  while (ch >= 48 && ch <= 57)
    x = (((x << 2) + x) << 1) + (ch & 15), ch = getchar();
  if (f) x = -x;
}
template <typename _Tp, typename... Args>
__inline__ __attribute__((always_inline)) void read(_Tp &t, Args &...args) {
  read(t);
  read(args...);
}
__inline__ __attribute__((always_inline)) int read_str(char *s) {
  register char ch(getchar());
  while (ch == ' ' || ch == '\r' || ch == '\n') ch = getchar();
  register char *tar = s;
  *tar = ch, ch = getchar();
  while (ch != ' ' && ch != '\r' && ch != '\n' && ch != EOF)
    *(++tar) = ch, ch = getchar();
  return tar - s + 1;
}
const int N = 100005;
long long h[N], a[N];
int n, m, k, p;
std::map<int, int> mp;
bool check(long long o) {
  mp.clear();
  for (int i = 1; i <= m; ++i) mp[i] = k;
  for (int i = 1; i <= n; ++i) {
    long long lim = o - a[i];
    int cur = m;
    while (cur > 1) {
      int l = 1, r = cur;
      while (l < r) {
        int mid = (l + r + 1) >> 1;
        if (lim - 1ll * a[i] * (cur - mid) < 0)
          l = mid;
        else
          r = mid - 1;
      }
      lim -= 1ll * a[i] * (cur - l);
      while (lim < 0) {
        auto it = mp.upper_bound(l);
        if (it == mp.end()) return false;
        if (!--it->second) mp.erase(it);
        lim += p;
      }
      cur = l;
    }
    lim -= h[i];
    while (lim < 0) {
      auto it = mp.begin();
      if (it == mp.end()) return false;
      if (!--it->second) mp.erase(it);
      lim += p;
    }
  }
  return true;
}
int main() {
  read(n, m, k, p);
  long long l = 0, r = 1e13;
  for (int i = 1; i <= n; ++i) {
    read(h[i], a[i]);
    chmax(l, a[i]);
  }
  while (l < r) {
    long long mid = (l + r) >> 1;
    if (check(mid))
      r = mid;
    else
      l = mid + 1;
  }
  printf("%lld\n", l);
  return 0;
}
