// EXPLOSION!
#define _CRT_SECURE_NO_WARNINGS
#include<bits/stdc++.h>
#include<unordered_set>
#include<unordered_map>
#include<chrono>

using namespace std;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pair<int, int>> vpi;
typedef vector<pair<ll, ll>> vpll;

#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)

#define pb push_back
#define mp make_pair
#define rsz resize
#define sz(x) int(x.size())
#define all(x) x.begin(),x.end()
#define f first
#define s second
#define cont continue
#define endl '\n'
//#define ednl '\n'
#define test int testc;cin>>testc;while(testc--)
#define pr(a, b) trav(x,a)cerr << x << b; cerr << endl;
#define message cout << "Hello World" << endl;
const int dx[4] = { 1,0,-1,0 }, dy[4] = { 0,1,0,-1 }; // for every grid problem!!
const ll linf = 4000000000000000000LL;
const ll inf = 1000000007;//998244353    

void pv(vi a) { trav(x, a)cout << x << " "; cout << endl; }void pv(vll a) { trav(x, a)cout << x << " "; cout << endl; }void pv(vector<vi>a) {
	F0R(i, sz(a)) { cout << i << endl; pv(a[i]); cout << endl; }
}void pv(vector<vll>a) { F0R(i, sz(a)) { cout << i << endl; pv(a[i]); }cout << endl; }void pv(vector<string>a) { trav(x, a)cout << x << endl; cout << endl; }
void setIO(string s) {
	ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef arwaeystoamneg
	if (sz(s))
	{
		freopen((s + ".in").c_str(), "r", stdin);
		if (s != "test1")
			freopen((s + ".out").c_str(), "w", stdout);
	}
#endif
}
int n, k;
vi a, c, dp;
vector<vi>ne;
int check(int m)
{
	fill(all(c), 0);
	F0R(i, k)ne[i][n] = inf;
	R0F(i, n)
	{
		F0R(j, k)c[j]++;
		if (a[i] != -1)
		{
			F0R(j, k)
			{
				if (j != a[i])c[j] = 0;
			}
		}
		F0R(j, k)
		{
			ne[j][i] = ne[j][i + 1];
			if (c[j] >= m)ne[j][i] = i;
		}
	}
	fill(all(dp), inf);
	dp[0] = 0;
	F0R(mask, 1 << k)
	{
		F0R(i, k)
		{
			if (mask & (1 << i))continue;
			int j = dp[mask];
			if (j < n)
			{
				j = ne[i][j];
				j += m;
				if (j <= n)
					dp[mask ^ (1 << i)] = min(dp[mask ^ (1 << i)], j);
			}
		}
	}
	return dp[(1 << k) - 1] < inf;
}
int main()
{
	setIO("test1");
	cin >> n >> k;
	string t;
	cin >> t;
	F0R(i, n)a.pb((t[i] == '?' ? -1 : t[i] - 'a'));
	ne.rsz(k, vi(n + 1));
	c.rsz(k);
	dp.rsz(1 << k);
	int l = 0, r = n;
	while (l < r)
	{
		int m = (l + r + 1) / 2;
		if (check(m))
		{
			l = m;
		}
		else
		{
			r = m - 1;
		}
	}
	cout << l << endl;
}
