import bisect import copy import decimal import fractions import heapq import itertools import math import random import sys import time from collections import Counter,deque,defaultdict from functools import lru_cache,reduce from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max def _heappush_max(heap,item): heap.append(item) heapq._siftdown_max(heap, 0, len(heap)-1) def _heappushpop_max(heap, item): if heap and item < heap[0]: item, heap[0] = heap[0], item heapq._siftup_max(heap, 0) return item from math import gcd as GCD read=sys.stdin.read readline=sys.stdin.readline readlines=sys.stdin.readlines N,M=map(int,readline().split()) A,B=map(int,readline().split()) dp0,dp1,dp2=-1<<60,-1<<60,max(B,B+A*(M-1),A*M) for i in range(N-1): A,B=map(int,readline().split()) dp0,dp1,dp2=max(dp0,dp1,dp2),max(dp1,dp2)+max(A,B),dp2+max(B,B+A*(M-1),A*M) ans=max(dp0,dp1,dp2) print(ans)