const processData = (lines) => {
  const r = +lines[0]
  console.log(r*Math.PI * 2)
}

let i = ''
process.stdin.on('data', c => i += c)
process.stdin.on('end', () => {
  const {EOL} = require('os')
  const lines = i.split(EOL) /*your input text, split by lines*/
  processData(lines)
})
