Adaptive form calculate expression for multiple instances

How do you call each instance and get the numeric value to calculate a total value? I'm stuck at identifying the syntax for each instance's value - here's the script for the calculated field:

var f = guideRootPanel.tblAgencies.Agency.instanceManager.instanceCount;
var total = 0;
for (var i=0; i < f; i++) {
total = total + guideRootPanel.tblAgencies.Agency(i).ContractID.value;
}
this.value = total;