import sys
input = sys.stdin.readline
n = int(input())
tm = 0
for _ in range(n):
h, m = map(int, input().split())
tm += h * 60 + m
mn = 10000000000000000
for i in range(20):
for j in range(20):
if i * 240 + j * 180 >= tm:
mn = min(mn, i * 1090 + j * 915)
print("{:.2f}".format(mn / 100))