import std.conv, std.functional, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.complex, std.container, std.math, std.numeric, std.range, 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;

int N, M;
long[] A, B;

long calc(int i, long x) {
  long ret;
  if (x == 0) {
    ret = 0;
  } else {
    ret = -INF;
    chmax(ret, A[i] * x);
    chmax(ret, A[i] * (x - 1) + B[i]);
    chmax(ret, B[i]);
  }
  return ret;
}

void main() {
  try {
    for (; ; ) {
      N = readInt();
      M = readInt();
      A = new long[N];
      B = new long[N];
      foreach (i; 0 .. N) {
        A[i] = readLong();
        B[i] = readLong();
      }
      
      auto DP = new long[][](N + 1, 3);
      foreach (i; 0 .. N + 1) {
        DP[i][] = INF;
      }
      DP[0][] = 0;
      foreach (i; 0 .. N) {
        DP[i + 1][0] = DP[i][0] + calc(i, 0);
        DP[i + 1][1] = DP[i][1] + calc(i, 1);
        DP[i + 1][2] = DP[i][2] + calc(i, M);
        if (i == 0) {
          DP[i + 1][0 .. 2] = -INF;
        }
        chmax(DP[i + 1][1], DP[i + 1][2]);
        chmax(DP[i + 1][0], DP[i + 1][1]);
      }
      
      debug {
        if (N <= 10 && M <= 10) {
          auto dp = new long[][](N + 1, M + 1);
          foreach (i; 0 .. N + 1) {
            dp[i][] = -INF;
          }
          dp[0][] = 0;
          foreach (i; 0 .. N) {
            foreach (j; 0 .. M + 1) {
              dp[i + 1][j] = dp[i][j] + calc(i, j);
            }
            if (i == 0) {
              dp[i + 1][0 .. M] = -INF;
            }
            foreach_reverse (j; 0 .. M) {
              chmax(dp[i + 1][j], dp[i + 1][j + 1]);
            }
          }
          foreach (i; 0 .. N + 1) {
            writeln(dp[i]);
          }
          assert(DP[N][0] == dp[N][0]);
        }
      }
      
      writeln(DP[N][0]);
    }
  } catch (EOFException e) {
  }
}