How many strings of five ASCII characters contain the
character @ (“at” sign) at least once? [Note: There are
128 different ASCII characters?

My wrong attempt:
Since there needs to be at least 1 @ symbol in every string we can set the 1st letter to @, then we have 128 options for the next letter and 128 options for the next letter again etc. So we have 1\*128^4 options.

Could someone explain why this doesn't work?