Prime Friend

Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 4659 Accepted Submission(s): 952

Problem Description

Besides the ordinary Boy Friend and Girl Friend, here we define a more academic kind of friend: Prime Friend. We call a nonnegative integer A is the integer B’s Prime Friend when the sum of A and B is a prime.
So an integer has many prime friends, for example, 1 has infinite prime friends: 1, 2, 4, 6, 10 and so on. This problem is very simple, given two integers A and B, find the minimum common prime friend which will make them not only become primes but also prime neighbor. We say C and D is prime neighbor only when both of them are primes and integer(s) between them is/are not.

Input

The first line contains a single integer T, indicating the number of test cases.
Each test case only contains two integers A and B.

Technical Specification

1. 1 <= T <= 1000
2. 1 <= A, B <= 150

Output

For each test case, output the case number first, then the minimum common prime friend of A and B, if not such number exists, output -1.

Sample Input

2 2 4 3 6

Sample Output

Case 1: 1 Case 2: -1

Author

iSea@WHU

Source

The 6th Central China Invitational Programming Contest and 9th Wuhan University Programming Contest Final

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#include<iostream>
#include<cstdio>
#include<string.h>
using namespace std;
#define N 21000000
bool a[N+5];
long long b[N+1];
int main()
{
   int T,i,j,k;
   long long A,B,e=1,t;
   memset(a,true,sizeof(a));
   a[0]=false;
   a[1]=false;
   for(i=2;i<N;i++)
   {
       if(a[i]==true)
       {
           for(j=i*2;j<N;j+=i)
           a[j]=false;
           b[e++]=i;
       }
   }
   k=1;
   scanf("%d",&T);
   while(T--)
   {
       t=-1;
       scanf("%lld%lld",&A,&B);
       if(A>B)
       swap(A,B);
       for(i=1;i<e;i++)
       {

           if(b[i]>=A&&b[i+1]>=B&&(b[i]-A==b[1+i]-B))
           {
               t=b[i]-A;
               break;
           }
       }
       cout<<"Case "<<k++<<": "<<t<<endl;
   }
   return 0;
}

素数朋友 prime friend

Prime Friend

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