#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <vector>
using namespace std;
using ll = long long;
#define rep(i, j, n) for (int i = j; i < (n); ++i)
#define rrep(i, j, n) for (int i = (n)-1; j <= i; --i)
#define all(a) a.begin(), a.end()
template <typename T>
std::ostream &operator<<(std::ostream &os, std::vector<T> &a) {
  for (size_t i = 0; i < a.size(); ++i) os << (i > 0 ? " " : "") << a[i];
  return os << '\n';
}
template <typename T>
std::istream &operator>>(std::istream &is, std::vector<T> &a) {
  for (T &x : a) { is >> x; }
  return is;
}

constexpr long long INFL = 0x3f3f3f3f3f3f3f3fLL;

ll n, d, p[1001], q[1001];

bool c(ll x) {
  ll val = 0, mae = -1;
  rep(_, 0, d) {
    ll nval = -INFL, nmae = -1;
    rep(i, 0, n) {
      if (mae == i) continue; // ここがびみょう
      // 可能な中で市場んvalがでかくなるの
      if (val - p[i] >= x) {
        ll tmp = val - p[i] + q[i];
        if (tmp > nval) {
          nval = tmp;
          nmae = i;
        }
      }
    }
    if (nval == -INFL) return false;
    val = nval;
    mae = nmae;
  }
  return true;
}

int main() {
  cin.tie(0)->sync_with_stdio(0);
  cin >> n >> d;
  rep(i, 0, n) cin >> p[i] >> q[i];

  ll l = -INFL, r = INFL;
  while (r - l > 1) {
    ll m = (l + r) / 2;
    if (c(m))
      l = m;
    else
      r = m;
  }
  cout << l << '\n';
  return 0;
}