n = int(input())
b = []

for _ in range(n):
    l, w = map(int, input().split())
    b.append((min(l,w), max(l, w)))

b.sort()

l = b[-1][1]
ans = b[-1][0] * b[-1][1] / 2.

for i in range(n-2, -1, -1):
    ans = max(ans, b[i][0] * min(l, b[i][1]), b[i][0] * b[i][1] / 2.)
    l = max(l, b[i][1])

print('{:.1f}'.format(ans))