n,m = map(int,input().split())
ab = [list(map(int,input().split())) for _ in range(n)]
X,Y,Z,W = [0]*4
for a,b in ab:
    x = a*m
    y = a*(m-1) + b
    X,Y,Z,W = X+max(x,y), max(X,Y)+max(a+b,b), max(X,Y,Z)+max(a,b), max(X,Y,Z,W)
print(max(X,Y,Z,W))