noname12 (NUMBER)

#include<bits/stdc++.h>
#define ll long long
#define pub(a, x) (a).push_back(x)
using namespace std;
void update(ll* n){
    *n=(*n<4)?((*n==2)?3:5):((*n-1)%6)?(*n+2):(*n+4);
}
bool cprm(ll n){
    if(n==2||n==3) return 1;
    if(n%2==0||n%3==0||n<2) return 0;
    for(ll i=5; i<=sqrt(n); update(&i)) if(n%i==0) return 0;
    return 1;
}
vector<ll> a={0}, b={0};
ll inp, n=1;
int main(){
    freopen("NUMBER.inp", "r", stdin);
    freopen("NUMBER.out", "w", stdout);
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin>>inp;
    pub(b, cprm(inp));
    while(cin>>inp){
        n++;
        if(cprm(inp)){
            pub(b, 1);
            if(b[n-2]){
                cout<<n-1;
                return 0;
            }
        }
        else pub(b, 0);
    }
    cout<<0;
}