private int Evaluate(string str) {
    var first = str[..3];
    var result = first == "one" ? 1 : 2;
    var index = 3;
    var lastOperation = '+';
    while (index < str.Length) {
        var substring = str.Substring(index);
        if (substring.StartsWith('+')) {
            lastOperation = '+';
            index++;
        } else if (substring.StartsWith('-')) {
            lastOperation = '-';
            index++;
        } else if (substring.StartsWith("one")) {
            result = lastOperation == '+' ? result + 1 : result - 1;
            index += 3;
        } else if (substring.StartsWith("two")) {
            result = lastOperation == '+' ? result + 2 : result - 2;
            index += 3;
        }
    }
    return result;
}

public static int Evaluate(string expression) {
    string processed = expression.Replace("one", "1").Replace("two", "2");
    var dataTable = new DataTable();
    var result = dataTable.Compute(processed, String.Empty);
    return Convert.ToInt32(result);
}

public static int Evaluate(string expression) {
    expression = expression.Replace("one", "1").Replace("two", "2");
    DataTable table = new DataTable();
    return Convert.ToInt32(table.Compute(expression, string.Empty));
}