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bool prime(int x) {
if (x == 2 || x == 3) return true;
if (x == 1 || x % 2 == 0 || x % 3 == 0) return false;
int k = -1;
while (k<=int(sqrt(x))) {
k+=6;
if (x % k == 0 || x % (k+2) == 0) break;
}
return k>int(sqrt(x));
}