n,m = map(int,input().split())
ab = [list(map(int,input().split())) for _ in range(n)]

INF = 10**18
X,Y,Z,V,W = [0]*5
for a,b in ab:
    x = a*m
    y = a*(m-1) + b
    X,Y,Z,V,W = max(X,Y)+x, max(X,Y)+y, max(X,Y,Z)+a, max(X,Y,Z,V)+b, max(X,Y,Z,V,W)
print(max(X,Y,Z,V,W))