#include<iostream>
bool isPrime(int x) {
if(x < 2) return false;
for(int i = 2; i < x; ++i) {
if(x % i == 0) return false;
}
return true;
}
int main() {
freopen("SNT.INP", "r", stdin);
freopen("SNT.OUT", "w", stdout);
std::string S;
std::cin >> S;
S = S + 'a';
int num = 0, res = -1;
for(int i = 0; i < S.length(); ++i) {
if('0' <= S[i] && S[i] <= '9') {
num = num * 10 + (S[i] - '0');
}
else {
if(isPrime(num)) res = std::max(res, num);
num = 0;
}
}
std::cout << res;
return 0;
}
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