import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 10L^^18;

void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const D = readInt();
      auto P = new long[N];
      auto Q = new long[N];
      foreach (i; 0 .. N) {
        P[i] = readLong();
        Q[i] = readLong();
      }
      
      bool check(long l) {
        debug {
          writeln("check ", l);
        }
        alias Entry = Tuple!(long, "val", int, "i");
        Entry[2] es = [Entry(0, -1), Entry(0, -1)];
        foreach (d; 0 .. D) {
          Entry[2] ees = [Entry(-INF, -1), Entry(-INF, -1)];
          foreach (i; 0 .. N) {
            const cur = es[(es[0].i != i) ? 0 : 1].val;
            if (cur - P[i] >= l) {
              auto tmp = Entry(cur - P[i] + Q[i], i);
              foreach (k; 0 .. 2) {
                if (ees[k] < tmp) {
                  swap(ees[k], tmp);
                }
              }
            }
          }
          debug {
            writeln("  ", ees);
          }
          es = ees;
        }
        return (es[0].val >= l);
      }
      
      long lo = -10^^9, hi = +1;
      for (; lo + 1 < hi; ) {
        const mid = (lo + hi) / 2;
        (check(mid) ? lo : hi) = mid;
      }
      writeln(lo);
    }
  } catch (EOFException e) {
  }
}