#include<bits/stdc++.h>
using namespace std;
bool ktsnt(long long n) {
if(n < 2) return false;
if(n == 2) return true;
for(long long i=2; i*i<=n; i++)
if(n % i == 0) return false;
return true;
}
bool isNumber(char ch) {
return (ch >= '0' && ch <= '9');
}
int main (){
string s;
getline(cin, s);
long long ans = 0;
for(int i=0; i<s.size(); i++) {
if(isNumber(s[i])) {
string so = "";
int j = i;
while(j < s.size() && isNumber(s[j])) {
so += s[j];
j++;
}
long long num = 0, sz = so.size();
long long d = 1;
for(int i=sz-1; i>=0; i--) {
num += 1LL*d*(so[i]-'0');
d = 1LL*d*10;
}
//cout << num << '\n';
if(ktsnt(num)) ans = max(ans, num);
i = j-1;
}
}
cout << ans << '\n';
}
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