Cartonase1

#include <fstream>
using namespace std;

int cmmdc(int a,int b)
{
    int r=a%b;
    while(r)
    {
        a=b;
        b=r;
        r=a%b;
    }
    return b;
}

int main()
{
    int n,a,b;
    ifstream f("cartonase1.in");
    f>>n>>a>>b;
    ofstream g("cartonase1.out");
    if(a==1)
    {
        g<<1;
        return 0;
    }
    int sol=cmmdc(a,b);
    if(sol==1)
    {
        g<<2;
        return 0;
    }
    for(int i=3;i<=n;++i)
    {
        a=b;
        f>>b;
        sol=cmmdc(sol,b);
        if(sol==1)
        {
            g<<i;
            return 0;
        }
    }
    g<<-1;
    return 0;
}