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 class WordNumberEvaluator {
    private const string ONE = "one";
    private const string TWO = "two";

    /// <summary>
    /// Evaluates a mathematical expression containing word numbers 'one' and 'two'.
    /// </summary>
    /// <param name="expression">The expression to evaluate</param>
    /// <returns>The numerical result of the expression</returns>
    public static int Evaluate(string expression) {
        // Replace word numbers with digits
        string processed = expression.Replace(ONE, "1").Replace(TWO, "2");
        // Use DataTable.Compute to evaluate the expression
        var dataTable = new DataTable();
        var result = dataTable.Compute(processed, String.Empty);
        return Convert.ToInt32(result);
    }
}

public class Program {
    public static void Main() {
        Console.WriteLine(WordNumberEvaluator.Evaluate("one+one"));          // Outputs: 2
        Console.WriteLine(WordNumberEvaluator.Evaluate("two-two-one-two"));  // Outputs: -3
        Console.WriteLine(WordNumberEvaluator.Evaluate("one+two-one-one+two+one")); // Outputs: 4
    }
}

using System;
using System.Data;

class Program {
    static void Main() {
        Console.WriteLine(Evaluate("one+one"));          // Output: 2
        Console.WriteLine(Evaluate("two-two-one-two"));  // Output: -3
        Console.WriteLine(Evaluate("one+two-one-one+two+one")); // Output: 4
    }

    public static int Evaluate(string expression) {
        // Replace "one" and "two" with their numeric equivalents
        expression = expression.Replace("one", "1").Replace("two", "2");
        // Use DataTable to evaluate the expression
        DataTable table = new DataTable();
        return Convert.ToInt32(table.Compute(expression, string.Empty));
    }
}