Sha 10710-Chinese shuffle (perfect shuffling)

Link to the question: Ultraviolet A 10710-Chinese shuffle

Question: give n cards, arrange them in order, perform the n-1 shuffling perfectly, and ask if they can be changed to the original German sequence.

Solution: according to the nature of the perfect shuffling, the X card after the P times has X? 2 P, so you only need to prove whether the 1st cards are in the distance.

#include <cstdio>#include <cstring>typedef long long ll;ll pow_mod(ll a, ll n, ll mod) {    ll ans = 1;    while (n) {        if (n&1)            ans = ans * a % mod;        a = a * a % mod;        n /= 2;    }    return ans;}int main () {    ll n;    while (scanf("%lld", &n) == 1 && n != -1) {        if (pow_mod(2, n-1, n) == 1)            printf("%lld is a Jimmy-number\n", n);        else            printf("%lld is not a Jimmy-number\n", n);    }    return 0;}