#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef complex<ld> cp;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pii> vii;
typedef vector<cp> vcp;
typedef vector<ld> vld;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<vii> vvii;

#define fastIO ios_base::sync_with_stdio(false), cin.tie(NULL)
#define forw(i, l, r) for( int i = (l) ; i < (r) ; i++ )
#define forb(i, r, l) for( int i = (r) ; i >= (l) ; i-- )
#define log2i(x) (64 - __builtin_clzll(1ll * (x)) - 1)
#define numBit(x) (__builtin_popcountll(1ll * (x)))
#define getBit(x, i) ((x) >> (i) & 1)
#define Pi acos(-1.0l)
#define sz(x) int(x.size())
#define mt make_tuple
#define mp make_pair
#define fi first
#define se second
#define pb push_back
#define pf push_front
#define pob pop_back
#define pof pop_front
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define debug(x) cerr << #x << " = " << x << '\n';

const int N = 2e5 + 7;
const int M = 17;
int n, k;
int dp[1 << M], suf[M][N], foccur[M][N];
string st;

bool kt(int x) {
    forw(i, 0, k) {
        foccur[i][n] = -1;
        forb(j, n - 1, 0) {
            if(j + x <= n && suf[i][j] - suf[i][j + x] == x) foccur[i][j] = j;
            else foccur[i][j] = foccur[i][j + 1];
        }
    }

    dp[0] = 0;
    forw(i, 1, 1 << k) {
        dp[i] = -1;
        forw(j, 0, k) if(getBit(i, j)) {
            int mask = i ^ (1 << j);
            if(dp[mask] == -1 || foccur[j][dp[mask]] == -1) continue;

            if(dp[i] == -1) dp[i] = foccur[j][dp[mask]] + x;
            else dp[i] = min(dp[i], foccur[j][dp[mask]] + x);
        }
    }
    return (dp[(1 << k) - 1] != -1);
}

int main() {
    fastIO;
#ifndef ONLINE_JUDGE
    freopen("test.inp","r",stdin);
    freopen("test.out","w",stdout);
#endif

    cin >> n >> k >> st;

    forw(i, 0, k) {
        suf[i][n] = 0;
        forb(j, n - 1, 0) suf[i][j] = suf[i][j + 1] + (st[j] == char(i + 'a') || st[j] == '?');
    }

    int l = 1, r = n / k;
    while(l < r) {
        int mid = (l + r + 1) / 2;
        if(kt(mid)) l = mid;
        else r = mid - 1;
    }
    cout << (kt(r) ? r : 0);
    return 0;
}
