#include <bits/stdc++.h> 
#define int long long 
#define all(a) (a).begin(),(a).end()
#define ti(a) ((int)a-'a')
#define tc(a) ((char)(a+'a')) 
#define sz(a) ((int)a.size())
#define endl '\n'
#define pb push_back
#define pii pair<int,int>
#define F first 
#define S second 
#define mp make_pair
using namespace std;
//DEBUG BEGIN 
#ifndef ONLINE_JUDGE 
    #define n_l '\n'
    #define dbg(...) cout << "[" << #__VA_ARGS__ << "]: "; cout << to_string(__VA_ARGS__) << endl
    template <typename T, size_t N> int SIZE(const T (&t)[N]){ return N; } template<typename T> int SIZE(const T &t){ return t.size(); } string to_string(const string s, int x1=0, int x2=1e9){ return '"' + ((x1 < s.size()) ? s.substr(x1, x2-x1+1) : "") + '"'; } string to_string(const char* s) { return to_string((string) s); } string to_string(const bool b) { return (b ? "true" : "false"); } string to_string(const char c){ return string({c}); } template<size_t N> string to_string(const bitset<N> &b, int x1=0, int x2=1e9){ string t = ""; for(int __iii__ = min(x1,SIZE(b)),  __jjj__ = min(x2, SIZE(b)-1); __iii__ <= __jjj__; ++__iii__){ t += b[__iii__] + '0'; } return '"' + t + '"'; } template <typename A, typename... C> string to_string(const A (&v), int x1=0, int x2=1e9, C... coords); int l_v_l_v_l = 0, t_a_b_s = 0; template <typename A, typename B> string to_string(const pair<A, B> &p) { l_v_l_v_l++; string res = "(" + to_string(p.first) + ", " + to_string(p.second) + ")"; l_v_l_v_l--; return res; } template <typename A, typename... C> string to_string(const A (&v), int x1, int x2, C... coords) { int rnk = rank<A>::value; string tab(t_a_b_s, ' '); string res = ""; bool first = true; if(l_v_l_v_l == 0) res += n_l; res += tab + "["; x1 = min(x1, SIZE(v)), x2 = min(x2, SIZE(v)); auto l = begin(v); advance(l, x1); auto r = l; advance(r, (x2-x1) + (x2 < SIZE(v))); for (auto e = l; e != r; e = next(e)) { if (!first) { res += ", "; } first = false; l_v_l_v_l++; if(e != l){ if(rnk > 1) { res += n_l; t_a_b_s = l_v_l_v_l; }; } else{ t_a_b_s = 0; } res += to_string(*e, coords...); l_v_l_v_l--; } res += "]"; if(l_v_l_v_l == 0) res += n_l; return res; } void dbgm(){;} template<typename Heads, typename... Tails> void dbgm(Heads H, Tails... T){ cout << to_string(H) << " | "; dbgm(T...); } 
    #define dbgm(...) cout << "[" << #__VA_ARGS__ << "]: "; dbgm(__VA_ARGS__); cout << endl
#endif 

// DEBUG END

//VECTOR BEGIN 
template<int D, typename T>
struct Vec : public vector<Vec<D - 1, T>> {
  static_assert(D >= 1, "Vector dimension must be greater than zero!");
  template<typename... Args>
  Vec(int n = 0, Args... args) : vector<Vec<D - 1, T>>(n, Vec<D - 1, T>(args...)) {
  }
};
template<typename T>
struct Vec<1, T> : public vector<T> {
  Vec(int n = 0, const T& val = T()) : vector<T>(n, val) {
  }
};
//VECTOR END
const int lg = 21 ;  
const int mxn = 102 ; 
const int md = 998244353 , nlg=60, inf = 1e18 ; 
int sqr(int a){return a*a;}
int msb(int x){ for(int i=nlg;i>-1;i--) if((1LL<<i)&x) return i ; return -1; }
int lsb(int x){ for(int i=0;i<nlg;i++) if((1LL<<i)&x) return i ; return -1;}
vector<int> tovec(string a){vector<int> ans ; for(char x:a) ans.pb(ti(x)); return ans;}
string tostr(vector<int> a){ string ans ; for(int x: a) ans+=tc(x); return ans;}
const int N = 1e6+1 ;
void solve(){
    int n , k ; 
    cin>>n>>k ; 
    string a ; 
    cin>>a ; 
    int l = 0 , r = n+1 ; 
    while(r-l!=1){
        int md = (r+l)/2 ; 
        int bgmask = (1LL<<k) ; 
        Vec<2,int> close(n,k,n) ; 
        vector<int> ans(bgmask,n) ; 
        for(int i=0;i<k;i++){
            int longest = 0 ; 
            for(int j=n-1;j>-1;--j){
                if(a[j]==tc(i)||a[j]=='?') ++longest ; 
                else longest = 0 ; 
                if(j+1<n) close[j][i]=close[j+1][i] ; 
                if(longest>=md)close[j][i]=j+md-1; 
            }
            ans[(1LL<<i)]=close[0][i] ; 
        }
        for(int mask = 0  ; mask <= bgmask ; ++mask){
            for(int i=0;i<k;i++){
                if((1LL<<i)&mask){
                    if(ans[(1LL<<i)^mask]<n-1){
                        ans[mask]=min(ans[mask],close[ans[(1LL<<i)^mask]+1][i]); 
                    }
                }
            }
        }
        if(ans[bgmask-1]<n) l = md ; 
        else r = md ; 
    }
    cout << l ; 
    return ; 
}
signed main(){
    ios_base::sync_with_stdio(0); 
    cin.tie(nullptr); 
    #ifndef ONLINE_JUDGE 
        freopen("in.txt","r",stdin); 
        freopen("out.txt","w",stdout); 
    #endif 

    int t=1; 
    while(t--){
        solve(); 
        cout << endl ; 
    }
    return 0 ; 
}