How do I remove error?

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C#

I declared a delegate using integer and methods using double and I get an error on main method where I instantiated the delegate object.how do I fix this?

What I have tried:

class Program
    {
        //creating a delegate Converter
        delegate int Converter(int value);

        //method to calculate Fahrenheit to Celsius
        static double ToCel(int fah)
        {
            return (fah - 32) / 1.8;
        }
        //method to callculate Celsius to Fahrenheit
        static double  ToFah(int celsius)
        {
            return celsius * 1.8 + 32;
        }
        //the method to calculate Celsius to Kelvin
        static double  ToKel(int celsius2)
        {
            return celsius2 + 273.15;
        }
        //the method to calculate celsius to Rankine
        static double ToRank(int celsius3)
        {
            return (celsius3 + 273.15) * 1.8;
        }
 static void Main(string[] args)
        {
            //institiating object for delegate

            Converter process = ToCel;
            Converter process1 = ToCel;
            Converter process2 = ToKel;
            Converter process3 = ToKel;
}
}