local ffi = require 'ffi'
local C = ffi.C

ffi.cdef [[
long atol(const char *);
int printf(const char *, ...);
]]

C.printf("%ld\n", (function (f, s)
	return f(s)
end)(function (s)
	local n = C.atol(s)
	return (n - 1LL) ^ 2LL / 4LL
end, io.stdin:read("*l")))