Error with calculator operands

I have a program that takes two values and adds them. I have an error in the controller, it tells me that it can not be converted from int to string. in this line: int result = Logica.Calculadora.Sumar(model.operando1, model.operando2) . What can it be?

Controller

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;


namespace Calculadora.MVC.Controllers
{
    public class CalculadoraController : Controller
    {
        // GET: Calculadora
        public ActionResult SumarConFormCollection()
        {
            return View();
        }

        [HttpPost]
        public ActionResult SumarConFormCollection(FormCollection form)
        {
            String op1Text = form["operando1"];
            String op2Text = form["operando2"];
            int result = Logica.Calculadora.Sumar(op1Text, op2Text);

            ViewBag.TextoResultado = "Resultado: " + result.ToString();

            return View();
        }

        public ActionResult SumarConModel()
        {
            return View();
        }
//método que hace la suma
        public ActionResult SumarConModel(Models.CalculadoraModel model)
        {
            int result = Logica.Calculadora.Sumar(model.operando1, model.operando2);

            model.TextoResultado = "Resultado: " + result.ToString();
            return View();
        }
    }
}

Model

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Calculadora.MVC.Models
{
    public class CalculadoraModel
    {
        public int operando1 { get; set; }

        public int operando2 { get; set; }

        public String TextoResultado { get; set; }
    }

}

View

@{
    ViewBag.Title = "SumarConModel";
}

<h2>SumarConModel</h2>

<form action="/calculadora/SumarConModel" method="post">
    <div>
        <label>Operando 1: </label>
        <input type="text" name="operando1" placeholder="Ingrese operando 1" />
    </div>

    <div>
        <label>Operando 2: </label>
        <input type="text" name="operando2" placeholder="Ingrese operando 2" />
    </div>

    <div>
        <input type="submit" name="sumar" value="Sumar" />
    </div>
</form>

@{
    if (@Model != null)
    {
        <div>
            <h2>@Model</h2>
        </div>
    }
}