Using Interface in plugins

UsingInterfaceInPlugins.cs

public void Execute(IServiceProvider serviceProvider)
{
    var context = serviceProvider.GetService(typeof(IPluginExecutionContext)) as IPluginExecutionContext;
    var serviceFactory = serviceProvider.GetService(typeof(IOrganizationServiceFactory)) as IOrganizationServiceFactory;
    var service = serviceFactory.CreateOrganizationService(context.UserId);

    var entityWithTotalSum = context.InputParameters["Target"] as IEntityWithTotalSum;
    entityWithTotalSum.new_totalsum = entityWithTotalSum.new_netamount + entityWithTotalSum.new_margin;          
}