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)