fx.lua

for _,expr in pairs({
    "1",
    "N",
    "N/2+1",
    "N/2+2",
    "math.random()"
}) do
    local fun,err = loadstring("return "..expr)
    if not fun then error("Bad expression '"..expr.."'") end
    local count = math.floor(math.random(7))
    setfenv(fun, { N = count, math = math })
    local ok, res = pcall(fun)
    if not ok then error("Failed to execute expression '"..expr.."': "..res) end
    res = math.floor(res)
    print(("For N=%s and expr='%s' got result %s"):format(count, expr, res))
end

For N=6 and expr='1' got result 1
For N=5 and expr='N' got result 5
For N=5 and expr='N/2+1' got result 3
For N=6 and expr='N/2+2' got result 5
For N=1 and expr='math.random()' got result 0