input: [Description] The number of times that the code for an interstitial ad (for example, Floating) is served by an ad server to a user's browser. This number can be greater than or equal to the number of impressions that are delivered. This is because, in some cases, the code for an interstitial ad is served to a user's browser but the ad isn't displayed. As a result, the code can be served without an impression's being counted. You can compare this metric to the number of delivered impressions to determine a play ratio. This metric is presented as a total number in reporting and isn't presented for standard ads like In-page or Expanding. [Metric] 
output: Code serves