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int greatestCommonPrimeDivisor(int a, int b) {
    int res = -1;
    for (int i = 2; i <= Math.max(a, b); i++) {
        if (a % i == 0 && b % i == 0 && p(i)) {
            if (res < i) res = i;
        }
    }
    return res;
}

boolean p(int a) {
    for (int i = 2; i < a; i++) {
        if (a % i == 0) return false;
    }
    return true;
}