n = int(input())
li = []
space = 0
for i in range(n):
    li.append(list(map(int, input().split())))

for i in li:
    if i[0] <= 2:
        space += i[1]
    elif not i[0] % 2 == 0:
        space += (i[0] // 2 + i[0] % 2) * i[1]
    else:
        space +- i[0] // 2 * i[1]

print(space)