const fs = require('fs');

fs.readFile('input', (err, data) => {
  if (err) throw err;
  let input = data.toString().trim().split("\n").map((line) => Number(line) );

  let builtIn = Math.max(...input) + 3;
  let adapters = new Set ( input );
  let jolts = 0;
  let diff1s = 0;
  let diff3s = 1; // The built-ion adapter

  for (i=0; i < input.length; i++) {
      // All entries are unique, so no need to worry about jolts+0
    if (adapters.has(jolts+1)) {
      diff1s++;
      adapters.delete(jolts+1);
      jolts++;
    } else if (adapters.has(jolts+2)) {
      adapters.delete(jolts+2);
      jolts+=2;
    } else if (adapters.has(jolts+3)) {
      diff3s++;
      adapters.delete(jolts+3);
      jolts+=3;
    } else {
      throw ("oh no");
    }
  };

  console.log(diff1s * diff3s);
});