n,m = map(int,input().split())
X,Y,Z = [0,-10**18,-10**18]
for _ in range(n):
    a,b = map(int,input().split())
    x = a*m
    y = a*(m-1) + b
    if _ == 0:
        X = max(x,y,b)
        continue
    X,Y,Z = X+max(x,y,b), max(X,Y)+max(a,b), max(X,Y,Z)
print(max(X,Y,Z))