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

Plus = []
Zero = []
Nega = []
for i in range(n):
    if P[i][0] <= P[i][1]:
        Plus.append(P[i])
    elif P[i][0] == P[i][1]:
        Zero.append(P[i])
    else:
        Nega.append(P[i])

if len(Plus) >= 2:
    Plus.sort(key=lambda x: x[0])
    print(min(-Plus[0][0], -Plus[0][0] + Plus[0][1] - Plus[1][0]))
elif len(Plus) == 1:
    if len(Zero) >= 1:
        Zero.sort(key=lambda x: x[0])
        print(max(-Plus[0][0], -Zero[0][0]))
    else:
        Nega.sort(key=lambda x: x[1] - x[0], reverse=True)
        print((-Nega[0][0] + Nega[0][1]) * (n // 2))
else:
    Nega.sort(key=lambda x: x[1] - x[0], reverse=True)
    print((-Nega[0][0] + Nega[0][1]) * (n - n // 2) + \
        (-Nega[1][0] + Nega[1][1]) * (n // 2))