export {};

const maxNum: number = 123456*2;
let isPrime: boolean[] = Array.from({length: maxNum + 1}, (_, i) => i>1 ? true : false);

for(let i=2; i*i <= maxNum; i++) {
    if (isPrime[i]) {
        let num: number = i*i;
        while(num <= maxNum) {
            isPrime[num] = false;
            num += i;
        }
    }
}

const input: number[] = require("fs").readFileSync(0).toString().trim().split('\n').map(Number);
for(let n of input) {
    if (n===0) break;
    let count: number = 0;
    for(let i=n+1; i<=2*n; i++) {
        if (isPrime[i]) count++;
    }
    console.log(count);
}