#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int FFTMOD = 119 << 23 | 1;
const int INF = (int)1e9 + 23111992;
const long long LINF = (long long)1e18 + 23111992;
const long double PI = acos((long double)-1);
const long double EPS = 1e-9;
inline long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
inline long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
inline int chkmin(T& a, const T& val) {
  return val < a ? a = val, 1 : 0;
}
template <class T>
inline int chkmax(T& a, const T& val) {
  return a < val ? a = val, 1 : 0;
}
inline unsigned long long isqrt(unsigned long long k) {
  unsigned long long r = sqrt(k) + 1;
  while (r * r > k) r--;
  return r;
}
inline long long icbrt(long long k) {
  long long r = cbrt(k) + 1;
  while (r * r * r > k) r--;
  return r;
}
inline void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
inline void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
inline int sign(long double x) { return x < -EPS ? -1 : x > +EPS; }
inline int sign(long double x, long double y) { return sign(x - y); }
mt19937 mt(chrono::high_resolution_clock::now().time_since_epoch().count());
inline int myrand() { return abs((int)mt()); }
const int maxn = 2e5 + 5;
int n, k;
long long a[maxn];
long long b[maxn];
long long c[maxn];
int check(long long mi) {
  memset(c, 0, sizeof(c));
  priority_queue<pair<int, pair<int, int>>> heap;
  for (int i = (0); i < (n); ++i) {
    long long t = a[i] / b[i] + 1;
    if (t <= k) {
      heap.push({-t, {b[i], i}});
    }
  }
  for (int i = (1); i < (k + 1); ++i) {
    if (int((heap).size())) {
      int ix = heap.top().second.second;
      heap.pop();
      c[ix] += mi;
      long long t = (a[ix] + c[ix]) / b[ix] + 1;
      if (t <= k) {
        heap.push({-t, {b[ix], ix}});
      }
    }
    if (int((heap).size()) && -heap.top().first == i) {
      return 0;
    }
  }
  return 1;
}
void chemthan() {
  cin >> n >> k, k--;
  for (int i = (0); i < (n); ++i) cin >> a[i];
  for (int i = (0); i < (n); ++i) cin >> b[i];
  long long lo = 0, hi = 1e18;
  while (lo < hi) {
    long long mi = lo + hi >> 1;
    if (!check(mi)) {
      lo = mi + 1;
    } else {
      hi = mi;
    }
  }
  if (lo == 1e18) lo = hi = -1;
  cout << (lo + hi >> 1) << "\n";
}
int main(int argc, char* argv[]) {
  ios_base::sync_with_stdio(0), cin.tie(0);
  if (argc > 1) {
    assert(freopen(argv[1], "r", stdin));
  }
  if (argc > 2) {
    assert(freopen(argv[2], "wb", stdout));
  }
  chemthan();
  cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
  return 0;
}
