private static double Calculate(string str)
{
List <double> result = new List <double>();
for (int i = 0; i < str.Length; i++)
{
if (char.IsDigit(str[i]))
{
string num = "";
while (char.IsDigit(str[i]))
{
num += str[i];
i++;
}
result.Add(Convert.ToDouble(num));
}
else if (Rpn.IsSign(str[i]))
{
double num1 = result[^ 1];