Why does my URL get processed before I press Enter? - asp.net-core

Question

I'm working through Adam Freeman's book "Pro ASP.Net Core 3" (8th edition). I am running an example of a page that uses information in a database. The Configure() method in my Startup class includes an endpoint for "/sum/{count:1000000000}". The base URL is http://localhost:5000. In Google Chrome, I type "http://localhost:5000/sum", and as soon as I type the "m", the web page is requested. If I want to get the calculation for 10 by requesting http://localhost:5000/sum/10, the page for 1000000000 is requested first. I can imagine that in a real-world application, that could end up being a problem. How do I avoid that?
Here's my startup file:
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Platform.Services;
using Microsoft.EntityFrameworkCore;
using Platform.Models;
using Microsoft.Extensions.Hosting;
using Microsoft.AspNetCore.Hosting;
namespace Platform {
public class Startup {
public Startup(IConfiguration config) {
Configuration = config;
}
private IConfiguration Configuration { get; set; }
public void ConfigureServices(IServiceCollection services) {
services.AddDistributedSqlServerCache(opts =>
{
opts.ConnectionString
= Configuration["ConnectionStrings:CacheConnection"];
opts.SchemaName = "dbo";
opts.TableName = "DataCache";
});
services.AddResponseCaching();
services.AddSingleton<IResponseFormatter, HtmlResponseFormatter>();
services.AddDbContext<CalculationContext>(opts =>
{
opts.UseSqlServer(Configuration["ConnectionStrings:CalcConnection"]);
});
services.AddTransient<SeedData>();
}
public void Configure(IApplicationBuilder app,
IHostApplicationLifetime lifetime, IWebHostEnvironment env,
SeedData seedData) {
app.UseDeveloperExceptionPage();
app.UseResponseCaching();
app.UseStaticFiles();
app.UseRouting();
app.UseEndpoints(endpoints => {
endpoints.MapEndpoint<SumEndpoint>("/sum/{count:int=1000000000}");
endpoints.MapGet("/", async context => {
await context.Response.WriteAsync("Hello World!");
});
});
bool cmdLineInit = (Configuration["INITDB"] ?? "false") == "true";
if (env.IsDevelopment() || cmdLineInit) {
seedData.SeedDatabase();
if (cmdLineInit) {
lifetime.StopApplication();
}
}
}
}
}