Calf problem

subject ： There is a cow on a farm , Now cows オー year , To be four years old オ Can have calves , After the age of four , Give birth to a calf every year . Suppose every time a cow is born , And also abide by 4 The principle of giving birth only after one year , And the cattle born will not die , ask 20 How many cows will there be after the year ？

int main(void){
    int i;
    int a[21];
    a[1] = 1;
    a[2] = 1;
    a[3] = 1;
    for (i=4;i<=20;i++)
        a[i] = a[i - 1] + a[i - 3];

    for (i=1;i<=20;i++)
        printf("%d ", a[i]);
    return 0;
}