using System;
using System.Text; 
public class Test
{
        public static void Main()
        {
StringBuilder stringBuilder = new StringBuilder();
string str = "+123-456/7890";
foreach(char c in str.ToCharArray() )
  if ( char.IsDigit(c) ) stringBuilder.Append(c);
 
str = stringBuilder.ToString();
 
                
        }
}