阅读程序填写结果：

#include <cstdio>
#include <iostream>
using namespace std;
int fun(int x) {
    if(x == 0 || x == 1)
        return 3;
    else
        return x + fun(x-2);
}
int main() {
    int k;
    cin >> k;
    printf("%d", fun(k));
    return 0;
}