ll f(string s,ll n,string o){
	ll a=0,b=n-1,z=0;
	while(a<b){
		while(a<n&&s[a]==o[z%3]) ++a;
		while(b>=0&&s[b]==o[z%3]) --b;
		++z;
	}
	return z+(a==b);
}

{
	ll@n;
	string@s;
	ll z=n;
	z<?=f(s,n,"RGB");
	z<?=f(s,n,"RBG");
	z<?=f(s,n,"GRB");
	z<?=f(s,n,"GBR");
	z<?=f(s,n,"BRG");
	z<?=f(s,n,"BGR");
	wt(z);
}