Find original

You are given a string $S_1$ which is made from combining all left cyclic rotations of another string $S_2$. Your task is to find the original string $S_2$. $ S_1$ = $d_0$ + $d_1$ + $d_2$ + .... + $d_{N-1}$, where $N$ is the length of $S_2$. $d_0$ is left rotation of $S_2$ by 0 elements, $d_1$ is left rotation of $S_2$ by 1 element, $d_2$ is left rotation of $S_2$ by 2 elements, and so on. ###