OJ Board

I have tested several inputs & got right answer. Then why am I getting WA . Plz someone help. Here is my code:

#include<iostream>
#include<cmath>
using namespace std;

#define n 100000
int a[n];
int pr[50000];

int main()
{
int i,j;

for(i=0;i<n;i++)
a[i]=0;

a[1]=1;

for(i=2;i<n;i++)
{
if ...

if everything is right still getting WA
try making the array size 10000 instead of 5002