n = int(input())
prev_t = 0
prev_v = 0
total = 0
for i in range(n):
    m = input().split()
    t = int(m[0])
    v = float(m[1])

    if prev_t == 0:
        prev_t = t
        prev_v = v
        continue

    total += ((prev_v + v) / 2) * (t - prev_t) / 1000
    prev_t = t
    prev_v = v

print(total)