// https://www.codingame.com/training/easy/brick-in-the-wall

// Node.js
// const readFileSync = require('fs').readFileSync;
// const inputFile = 'lib/1.in';
// const inArray = readFileSync(inputFile, 'utf8').split('\n');
// const readline = () => inArray.shift();

const X = +readline();
readline();

const weights = readline()
  .split(' ')
  .map(Number)
  .sort((a, b) => a - b);

const total = weights.reduceRight(
  ({ total, floor, bricksInRow }, currWeight) => {
    if (bricksInRow === X) {
      floor++;
      bricksInRow = 0;
    }
    bricksInRow++;
    total += (((floor - 1) * 6.5) / 100) * 10 * currWeight;
    return { total, floor, bricksInRow };
  },
  { total: 0.0, floor: 1, bricksInRow: 0 }
).total;

console.log(total.toFixed(3));