#include <stdio.h>
 
unsigned long long product(int n) {
    unsigned long long result = 1;
    for (int i = 1; i <= n; i++) {
        result *= i;
    }
    return result;
}
 
int main() {
    int n;
 
    printf("n を入力してください（0以上の整数）: ");
    scanf("%d", &n);
 
    if (n < 0) {
        printf("エラー: 0以上の整数を入力してください。\n");
        return 1;
    }
 
    unsigned long long result = product(n);
    printf("1から%dまでの積（%d!）は %llu です。\n", n, n, result);
 
    return 0;
}

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NQ==

5