kekc

#include <stdio.h>
#include <malloc.h>
#include <stdint.h>

const uint64_t del = 1e9;

int main()
{
    int n = 0;
    scanf("%d", &n);

    uint64_t* nums = malloc(n * sizeof(uint64_t));
    for (uint64_t* i = nums; i != nums + n; i++)
        scanf("%llu", i);

    uint64_t** memo = malloc(n * sizeof(uint64_t*));
    for (uint64_t** i = memo; i != memo + n; i++)
        *i = calloc(n, sizeof(uint64_t));

    for (int i = 0; i < n; i++)
        memo[i][i] = 1;

    for (int r = 1; r < n; r++)
    {
        for (int i = 0, q = r; q < n; i++, q++)
        {
            memo[i][q] = (memo[i + 1][q] % del + memo[i][q - 1] % del - memo[i + 1][q - 1] % del) % del;

            if (nums[i] == nums[q])
                memo[i][q] = (memo[i][q] % del + memo[i + 1][q - 1] % del + 1) % del;
        }
    }

    printf("%llu",  memo[0][n - 1] % del);

    for (uint64_t** i = memo; i != memo + n; i++) free(*i);
    free(memo); free(nums);
    return 0;
}