const println = (value) => console.log(value)
const readInt = (input) => parseInt(input, 10)
const readIntList = (input) => input.split(' ').map(n => parseInt(n, 10))

function main(input) {
    const [n, d] = readIntList(input)
    let ans = 1
    while (true) {
        if (ans * 2 * d + ans  >= n) {
            break
        }
        ans++
    }
    println(ans)
}

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