/*input
3
aba

*/
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long,null_type,less_equal<long long>,rb_tree_tag,tree_order_statistics_node_update> indexed_set;
#pragma GCC optimize("O2","unroll-loops","no-stack-protector")
//order_of_key #of elements less than x
// find_by_order kth element
using ll=long long;
using ld=long double;
using pii=pair<ld,ld>;
#define f first
#define s second
#define pb push_back
#define REP(i,n) for(ll i=0;i<n;i++)
#define REP1(i,n) for(ll i=1;i<=n;i++)
#define FILL(n,x) memset(n,x,sizeof(n))
#define ALL(_a) _a.begin(),_a.end()
#define sz(x) (int)x.size()
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()),c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
const ll INF64=1e18+1;
const int INF=0x3f3f3f3f;
const ll MOD=1e9+7;
const ld PI=acos(-1);
const ld eps=1e-3;
#define lowb(x) x&(-x)
#define MNTO(x,y) x=min(x,(__typeof__(x))y)
#define MXTO(x,y) x=max(x,(__typeof__(x))y)
inline ll mult(ll a,ll b){
    if(a>=MOD) a%=MOD;
    if(b>=MOD) b%=MOD;
    return (a*b)%MOD;
}
inline ll mypow(ll a,ll b){
    if(b<=0) return 1;
    ll res=1LL;
    while(b){
        if(b&1) res=mult(res,a);
        a=mult(a,a);
        b>>=1;
    }
    return res;
}
const int maxn=1e4+5;
const int maxlg=__lg(maxn)+2;
int32_t main(){
    ios::sync_with_stdio(false),cin.tie(0);
    int n;
    string s;
    cin>>n>>s;
    if(s[0]!=s[n-1]) cout<<1;
    else{
        REP(i,n-1){
            if(s[i]!=s[0] and s[i+1]!=s[0]){
                cout<<2;
                return 0;
            }
        }
        cout<<-1;
    }
}
#include<bits/stdc++.h>

int main(){
    using namespace std;
    unsigned long N;
    cin >> N;
    string S;
    cin >> S;
    if(S[0] != S.back())return 0 & puts("1");
    for(unsigned long i{1}; i < N; ++i)if(S[0] != S[i - 1] && S[i] != S.back())return 0 & puts("2");
    puts("-1");
    return 0;
}
