// Original Author: Iaroslav Lazunov

aDoc = app.activeDocument;
aSel = aDoc.selection;
nSel = aSel.length;

opMin = Number(prompt("min (0-100)", 0));
opMax = Number(prompt("max (0-100)", 100));

if (opMin > opMax) {
  temp = opMin;
  opMin = opMax;
  opMax = temp;
}
if (opMin < 0) opMIn = 0;
if (opMax > 100) opMax = 100;

for (i = 0; i < nSel; i++) {
  aSel[i].opacity = Math.floor(Math.random() * (opMax - opMin + 1)) + opMin;
}