求思路，完全没有一点想法代码该怎么写？

#include <stdio.h>

int foo(int seed, int remain)
{
    if (seed == 0 && remain == 0) return 1;
    if (remain == 0) return 0;
    if (seed > 0)
        return foo(seed + 1, remain - 1) + foo(seed - 1, remain - 1);
    else
        return foo(seed + 1, remain - 1);
}

int main()
{
    int n;
    scanf("%d", &n);
    int result = foo(0, n * 2);
    printf("%d", result);
    return 0;
}

4
14Press any key to continue . . .
所以是14