'use strict'

function main(s) {

  const num = Number(s);
  const ary = [2, 1];

  for (var i = 2; i <= 86; i++) {
    ary[i] = ary[i - 1] + ary[i - 2];
  }

  const answer = ary[num];

  console.log(answer);
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));
