local n = io.read("*n")
local a, b = {}, {}
for i = 1, n do
  a[i], b[i] = io.read("*n", "*n")
end
for i = 2, n do
  a[i], b[i] = a[i] - a[1], b[i] - b[1]
end
local ret = 0
for i = 2, n - 1 do
  ret = ret + math.abs(a[i] * b[i + 1] - a[i + 1] * b[i])
end
print(ret)