#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const long long linf = 0x3f3f3f3f3f3f3f3f;
const int maxn = 2e5 + 11;
const int maxm = 600 + 11;
const int mod = 1e9 + 7;
const double eps = 1e-5;
inline long long rd() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
inline long long qpow(long long a, long long b, long long p) {
  long long res = 1;
  while (b) {
    if (b & 1) {
      res *= a;
      res %= p;
    }
    b >>= 1;
    a = a * a % p;
  }
  return res;
}
inline long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
struct node {
  long long a, b, c;
  bool operator<(const node &o) const {
    if (c != o.c) return c > o.c;
    if (b != o.b) return b < o.b;
    return a > o.a;
  }
} t[maxn];
int n, k;
bool check(long long x) {
  priority_queue<node> q;
  for (int i = 1; i <= n; i++) {
    q.push(t[i]);
  }
  for (int i = 0; i < k; i++) {
    node o = q.top();
    q.pop();
    if (o.c < i) return false;
    if (o.c >= k) return true;
    o.a += x;
    o.c = o.a / o.b;
    q.push(o);
  }
  return true;
}
int main() {
  n = rd(), k = rd();
  for (int i = 1; i <= n; i++) {
    t[i].a = rd();
  }
  for (int i = 1; i <= n; i++) {
    t[i].b = rd();
    t[i].c = t[i].a / t[i].b;
  }
  long long l = 0, r = 5e12, ans = -1;
  while (l <= r) {
    long long mid = l + r >> 1;
    if (check(mid))
      ans = mid, r = mid - 1;
    else
      l = mid + 1;
  }
  printf("%lld\n", ans);
}
