#include<bits/stdc++.h>
using namespace std;
int main()
{
    freopen("liars.in","r",stdin);
    freopen("liars.out","w",stdout);
    int n;
    cin>>n;
    string s;
    cin>>s; bool f=1;
    if(s[0]=='S')
    {
        if(s[n-1]!='L'&&s[1]!='S') f=0;
    }
    else if(s[n-1]!='S'&&s[1]!='L') f=0;
    if(s[n-1]=='S')
    {
        if(s[n-2]!='L'&&s[0]!='S') f=0;
    }
    else if(s[n-2]!='S'&&s[0]!='L') f=0;
    for(int i=1;i<n-1;i++)
    {
        if(s[i]=='S')
        {
            if(s[i-1]!='L'&&s[i+1]!='S'){f=0; break;}
        }
        else if(s[i-1]!='S'&&s[i+1]!='L'){f=0; break;}
    }
    if(f) cout<<"OK"<<endl;
    else cout<<"INVALID"<<endl;
    return 0;
}