One-to-one relationship, entity framework

public class Route
    {

        [Key]
        public int routeID { get; set; }
        public string Name { get; set; }

        public int? UserProfileID { get; set; }
        public int? CountryID { get; set; }
        //public int? DifficultGradeID { get; set; }



        public virtual UserProfile userProfile { get; set; }
        public virtual Country country { get; set; }
        public virtual Difficult difficult { get; set; }
    }



public class Difficult
    {
        [Key, ForeignKey("Route")]
        public int routeID { get; set; }
        public string DifficultName { get; set; }

        public virtual Route route { get; set; }

    }

Quote:

The ForeignKeyAttribute on property 'routeID' on type 'ContosoUniversity.Models.Difficult' is not valid. The navigation property 'Route' was not found on the dependent type 'ContosoUniversity.Models.Difficult'. The Name value should be a valid navigation property name.