#include <stdio.h>
 
int rec(int n) {
    // 0以下になったら終了（偶数の場合は2->0となりここで終了する）
    if (n <= 0) {
        return 0;
    } 
    // 1になったら1を返す（奇数の終了条件）
    else if (n == 1) {
        return 1;
    } 
    // それ以外は n を足して 2 引いた値を再帰呼び出し
    else {
        return rec(n - 2) + n;
    }
}
 
int main(void) {
    int n = 11;
 
    // n が偶数か奇数かでメッセージを変える
    if (n % 2 == 0) {
        printf("%d以下の正の偶数の和は%d\n", n, rec(n));
    } else {
        printf("%d以下の正の奇数の和は%d\n", n, rec(n));
    }
 
    return 0;
}

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