C++ :

#include <bits/stdc++.h>
using namespace std;

bool sushu(int n){
	int i;
	if(n <= 1) return false; 
	for(i = 2;i <= sqrt(n);i++){
		if(n % i == 0){
			return false;
		}
	}
	return true;
}

int main(){
	int n,i,c = 0,ma = 0;
	cin>>n;
	for(i = 2;i <= n;i++){
		if(sushu(i) == false){
			c++;
		}else{
			if(c > ma){
				ma = c;
			}
			
			c = 0;
		}
		
		if(i == n) ma = max(c,ma);
	}
	
	cout<<ma;
}

Java :

import java.util.Arrays;
import java.util.Scanner;

public class Main {
	//素数表
	static boolean a[] = new boolean[1000001];
	//记录连续最大数
	static int b[] = new int[1000001];
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		//将数组元素都设为true
		Arrays.fill(a, true);
		int i,j,n=sc.nextInt();
		//1 的情况定性
		a[1] = false;
		b[1] = 1;
		//从2开始素数表
		for(i=2;i<=n;i++) {
			for(j=2;j<=Math.sqrt(i);j++) {
				if(i%j==0) {
					a[i] = false;
					break;
				}
			}
			//a[i]要么false  要么true
			if(a[i]==false) {
				if(a[i-1]==false) b[i] = b[i-1] + 1;
				else b[i] = 1;
			}
		}
		int mx = Integer.MIN_VALUE;
		for(i=1;i<=n;i++) {
			mx = Math.max(b[i],mx);
		}
		System.out.println(mx);
		sc.close();
	}
}