I am having trouble in following program

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C

algorithm

The question is as follow--
A string contains only 0's and 1's;
Given n(length of the string) find how many permutations of string are possible where every prime length contagious subsequence of the string has greater or equal number of 0's than number of 1's.

My approach--
I have followed a pattern such that for n=2 answer is 3,for n=3 the answer is 4,n=4 answer=6,n=5 answer=9....
But my answer is not correct.
Following is my c code.

#include <stdio.h>

int main()
{
    
    
    
        long int n,i;
        scanf("%ld",&n);
        if(n<3)
        {
           
         if(n=2)
            printf("3");
        }
        else
        {
        long int a[n];
        
        a[0]=3;
        for(i=1;i<n-1;i++)
        a[i]=a[i-1]+i;
        long int temp=a[n-2]%1000000007;
        printf("%ld",temp);
        }
        printf("\n");
   
    return 0;
}