public int[] convo(int[] a, int[] b, int n)
{
   int[] c = new int[a.length];
   for(int i =0; i<n-1; i++)
   {
      for(int j=0;j<n;j++)
      {
      int k = (i+j)%n;
      c[k] += a[i]*b[j];
      }
   }
   return c;
}