#include <bits/stdc++.h>
#define mp make_pair
#define pb push_back
#define X first
#define Y second
#define y0 y12
#define y1 y22
#define INF 987654321
#define PI 3.141592653589793238462643383279502884
#define fup(i,a,b,c) for(int (i)=(a);(i)<=(b);(i)+=(c))
#define fdn(i,a,b,c) for(int (i)=(a);(i)>=(b);(i)-=(c))
#define MEM0(a) memset((a),0,sizeof(a))
#define MEM_1(a) memset((a),-1,sizeof(a))
#define ALL(a) a.begin(),a.end()
#define COMPRESS(a) sort(ALL(a));a.resize(unique(ALL(a))-a.begin())
#define SYNC ios_base::sync_with_stdio(false);cin.tie(0)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef double db;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int, int> Pi;
typedef pair<ll, ll> Pll;
typedef pair<ld, ld> Pd;
typedef vector<int> Vi;
typedef vector<ll> Vll;
typedef vector<ld> Vd;
typedef vector<Pi> VPi;
typedef vector<Pll> VPll;
typedef vector<Pd> VPd;
typedef tuple<int, int, int> iii;
typedef tuple<int, int, int, int> iiii;
typedef tuple<ll, ll, ll> LLL;
typedef vector<iii> Viii;
typedef vector<LLL> VLLL;
typedef complex<double> base;
const int MOD = 1000000007;
ll POW(ll a, ll b, ll MMM = MOD) { ll ret = 1; for (; b; b >>= 1, a = (a*a) % MMM)if (b & 1)ret = (ret*a) % MMM; return ret; }
int dx[] = { 0,1,0,-1,1,1,-1,-1 }, dy[] = { 1,0,-1,0,1,-1,1,-1 };
int ddx[] = { -1,-2,1,-2,2,-1,2,1 }, ddy[] = { -2,-1,-2,1,-1,2,1,2 };

char s[100005];
int d[100005],d2[26];
int main(){
	int n;
	scanf("%d%s",&n,s);
	fup(i,0,n-1,1)d[i]=INF;
	fup(i,0,25,1)d2[i]=INF;
	fdn(i,n-2,0,1){
		if(s[i]!=s[n-1])d[i]=1;
		fup(j,0,25,1){
			if(s[i]-'a'!=j)d[i]=min(d[i],1+d2[j]);
		}
		if(i>0)d2[s[i-1]-'a']=min(d2[s[i-1]-'a'],d[i]);
	}
	printf("%d\n",d[0]==INF?-1:d[0]);
}
#include <bits/stdc++.h> 
using namespace std;

template <class T>
inline bool mnto(T& a, T b) {return a > b ? a = b, 1 : 0;}
template <class T>
inline bool mxto(T& a, T b) {return a < b ? a = b, 1: 0;}
#define REP(i, s, e) for (int i = s; i < e; i++)
#define RREP(i, s, e) for (int i = s; i >= e; i--)
typedef long long ll;
typedef long double ld;
#define MP make_pair
#define FI first
#define SE second
typedef pair<int, int> ii;
typedef pair<ll, ll> pll;
#define MT make_tuple
typedef tuple<int, int, int> iii;
#define ALL(_a) _a.begin(), _a.end()
#define pb push_back
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<ii> vii;

#ifdef DEBUG
#define debug(args...) _debug(args)
void _debug(const char* format, ...) {
	va_list args;
	va_start(args, format);
	vprintf(format, args);
	va_end(args);
}
#else
#define debug(args...)
#endif

#define INF 1000000005
#define LINF 1000000000000000005
#define MOD 1000000007
#define MAXN 200005

int n;
char s[MAXN];

int main() {
	scanf("%d", &n);
	scanf(" %s", s);
	if (s[0] != s[n - 1]) {
		printf("1\n");
		return 0;
	}
	REP (i, 2, n - 1) {
		if (s[i - 1] != s[0] && s[i] != s[0]) {
			printf("2\n");
			return 0;
		}
	}
	printf("-1\n");
	return 0;
}
