let readline = require('readline')
const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
})

// 单行输入
rl.on('line', function (line) {

    function isPrime(num) { 

        if (num === 1) { return false; }

        for (let i = 2, len = num; i * i <= len; i++) {
            if (num % i === 0) { return false }
        }
        return true;
    }

    let nums = line.split(' ')

    let x = Math.min(Number(nums[0]), Number(nums[1]));
    let y = Math.max(Number(nums[0]), Number(nums[1]));
    let sum = 0

    for (let i = x, len = y; i <= len; i++)
        if (isPrime(i))
            sum++;

    console.log(sum)

    rl.close()
})