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OI-codes/XJOI/contest/21novice/D/D.cpp

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#include <bits/stdc++.h>
using namespace std;
int t, a, cnt, prime[100005];
bool flag, is_prime[1000005];
int eratosthenes(int n) {
int p = 0;
for (int i = 0; i <= n; i++) is_prime[i] = 1;
is_prime[0] = is_prime[1] = 0;
for (int i = 2; i <= n; i++) {
if (is_prime[i]) {
prime[p++] = i;
if (1ll * i * i <= n) {
for (int j = i * i; j <= n; j += i) {
is_prime[j] = 0;
}
}
}
}
return p;
}
long long binpow(long long a, long long b, long long m) {
a %= m;
long long res = 1;
while (b > 0) {
if (b & 1) res = res * a % m;
a = a * a % m;
b >>= 1;
}
return res;
}
int main() {
cin >> t;
cnt = eratosthenes(1000000);
while (t--) {
flag = false;
cin >> a;
for (int i = 0; i < cnt; i++) {
if (binpow(a, prime[i], 10) % 10 == binpow(prime[i], a, 10) % 10) {
cout << prime[i] << endl;
flag = true;
break;
}
}
if (!flag) cout << -1 << endl;
}
return 0;
}