const fs = require('fs');

// parameter "input" gets all data
function Main(input) {
    const r = parseInt(input);
    // const c = 2 * Math.PI * r;
    const c = 6283 * r;

    //output
    console.log(c / 1000);
}

// Don't edit this line!
Main(fs.readFileSync('/dev/stdin', 'utf8'));
