How many times do I need to try something that has an `odds` probability of winning to ensure with a certain level of `confidence` that I'll win.

probs.js

function rolls(odds, confidence) {
  var prob = 0;
  for (var n = 0; prob < confidence;) {
    n++;
    prob += calcProb(1, odds) * calcProb(n-1, 1-odds);
  }
  return n;
}

function calcProb(times, odds) {
  return Math.pow(odds, times);
}