Mix 2 strings that do not have the same length?

I’m trying to mix two strings like this:

string a1 = "123456";
string a2 = "DFEG";

var builder = new StringBuilder();

for (int i = 0; i < a1.Length; i++)
{
    builder.Append(a1[i]);
    builder.Append(a2[i]);
}

Console.WriteLine(builder.ToString());

I want the output to be "1D2F3E4G56".

It works if both strings are the same lengths, but not if they’re different lengths.

>Solution :

 int maxIndex = a1.Length > a2.Length ? a1.Length : a2.Length;
 for (int i = 0; i < maxIndex; i++)
 {
   if(a1.Length > i)
     builder.Append(a1[i]);
            
   if(a2.Length > i)
     builder.Append(a2[i]);
 }