題目←

每個數n的最小質因子一定不超過n√
預處理出m−−√內的素數表，刪去每個素數在n-m區間內所有倍數，剩下的一定全為素數
del陣列採用下標偏移，i實際表示數n+i是否已被刪去
del陣列大小為0~m-n

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#define LL long long
using namespace std;
const LL MAXN = 1000000 + 50;
LL prime[MAXN];
bool sh[MAXN],del[MAXN];
int
 
 cnt;
void solve1(LL n){
    for(LL i = 2;i <= n;i ++){
        if(!sh[i])prime[++ cnt] = i;
        for(LL j = 1;j <= cnt&&i*prime[j] <= n;j ++){
            sh[i*prime[j]] = true;
            if(i%prime[j] == 0)break;
        }
    }
}
int tot;
void solve2(LL n,LL m){
    for(LL i = 1
 
;i <= cnt;i ++){
        LL j = n/prime[i];
        if(j*prime[i] < n)j ++;
        if(j == 1)j ++;
        for(j;j*prime[i] <= m;j ++){
            if(!del[j*prime[i] - n])tot ++;
            del[j*prime[i] - n] = true;
        }
    }
}
LL n,m;
int main(){
    scanf("%lld%lld",&n,&m);
    LL t = sqrt
 
(m) + 1;
    solve1(t);
    solve2(n,m);
    printf("%d",m - n + 1 - tot);
}