#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int DY[]{1, 0, -1, 0}, DX[]{0, -1, 0, 1};
constexpr int DY8[]{1, 1, 0, -1, -1, -1, 0, 1}, DX8[]{0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n, d; cin >> n >> d;
  vector<int> p(n), q(n); REP(i, n) cin >> p[i] >> q[i];
  int lb = d * -*max_element(ALL(p)), ub = 0;
  while (ub - lb > 1) {
    int least = (lb + ub) / 2, power = 0, prev = -1;
    bool is_valid = true;
    REP(_, d) {
      int cand = -1;
      REP(i, n) {
        if (i == prev) continue;
        if (power - p[i] >= least) {
          if (cand == -1) {
            cand = i;
            continue;
          }
          int up = q[i] - p[i];
          if (up > q[cand] - p[cand]) {
            cand = i;
          } else if (up == q[cand] - p[cand]) {
            if (p[i] > p[cand]) cand = i;
          }
        }
      }
      if (cand == -1) {
        is_valid = false;
        break;
      }
      power += q[cand] - p[cand];
      prev = cand;
    }
    (is_valid ? lb : ub) = least;
  }
  cout << lb << '\n';
  return 0;
}