#include <bits/stdc++.h>
using namespace std;
struct InputReader {
  char buf[1000001];
  int p;
  inline InputReader() { p = 1000001; }
  inline void Flush() {
    p = 0;
    fread(buf, 1, 1000001, stdin);
  }
  inline char C() {
    if (p >= 1000001) Flush();
    return buf[p++];
  }
  inline char Readnum() {
    char ch = C();
    while (!isdigit(ch) && ch != '-') ch = C();
    return ch;
  }
  inline void Readalpha(char &c) {
    c = C();
    while (!isalpha(c)) c = C();
  }
  int operator()() {
    int ans = 0, fu = 1;
    char ch = Readnum();
    if (ch == '-') fu = -1, ch = C();
    while (ch >= '0' && ch <= '9') {
      ans = ans * 10 + ch - '0';
      ch = C();
    }
    return ans * fu;
  }
  long long Readll() {
    long long ans = 0LL, fu = 1LL;
    char ch = Readnum();
    if (ch == '-') fu = -1LL, ch = C();
    while (ch >= '0' && ch <= '9') {
      ans = ans * 10LL + ch - '0';
      ch = C();
    }
    return ans * fu;
  }
  inline void Readstring(string &x) {
    x.clear();
    char ch = C();
    while (!isdigit(ch) && !isalpha(ch) && ch != '#' && ch != '.') ch = C();
    while (isdigit(ch) || isalpha(ch) || ch == '#' || ch == '.') {
      x += ch;
      ch = C();
    }
  }
  inline void Readchstring(char s[]) {
    int len = 0;
    char ch = C();
    while (!isdigit(ch) && !isalpha(ch)) ch = C();
    while (isdigit(ch) || isalpha(ch)) {
      s[len++] = ch;
      ch = C();
    }
    s[len] = '\0';
  }
  inline void Specialread(char &c) {
    c = C();
    while (!isdigit(c) && !isalpha(c) && c != '#' && c != '.' && c != '=' &&
           c != 'B')
      c = C();
  }
} In;
inline void Read(int &x) { x = In(); }
inline void Read(int &x, int &y) {
  x = In();
  y = In();
}
inline void Read(int &x1, int &x2, int &x3) {
  x1 = In();
  x2 = In();
  x3 = In();
}
inline void Read(int &x1, int &x2, int &x3, int &x4) {
  x1 = In();
  x2 = In();
  x3 = In();
  x4 = In();
}
inline void Read(long long &x) { x = In.Readll(); }
inline void Read(long long &x, long long &y) {
  x = In.Readll();
  y = In.Readll();
}
inline void Read(long long &x1, long long &x2, long long &x3) {
  x1 = In.Readll();
  x2 = In.Readll();
  x3 = In.Readll();
}
inline void Read(long long &x1, long long &x2, long long &x3, long long &x4) {
  x1 = In.Readll();
  x2 = In.Readll();
  x3 = In.Readll();
  x4 = In.Readll();
}
inline void FILEIO() {}
inline void FILEIO(string pname) {
  freopen((pname + ".in").c_str(), "r", stdin);
  freopen((pname + ".out").c_str(), "w", stdout);
}
void Printtime() {}
void END() {
  Printtime();
  exit(0);
}
template <typename T>
void END(T mes) {
  cout << mes << endl;
  END();
}
template <typename T>
void Print(T a[], int s, int t, char sp = ' ', char ed = '\n') {
  if (s > t) return;
  for (int i = s; i < t; i++) cout << a[i] << sp;
  cout << a[t] << ed;
  cout.flush();
}
template <typename T>
void Print(T a, int s = 0, int t = -1, char sp = ' ', char ed = '\n') {
  if (t == -1) t = a.size() - 1;
  for (int i = s; i <= t; i++) cout << a[i] << sp;
  cout << ed;
  cout.flush();
}
int p, n;
int a[100010];
int can[17][17];
int cnt[17];
bool ban[(1 << 17)];
bool vi[(1 << 17)][17][17];
void Addban(int mask, int a, int b) {
  if (vi[mask][a][b]) return;
  vi[mask][a][b] = 1;
  ban[mask] = 1;
  for (int(i) = 0; (i) < (p); ++(i)) {
    if (i == a or i == b) continue;
    if (((mask) >> (i)&1)) continue;
    Addban(((mask) | 1 << (i)), a, b);
  }
}
int ans;
bool vis[(1 << 17)];
void Dfs(int mask, int rem) {
  if (vis[mask] or ban[mask]) return;
  vis[mask] = 1;
  (ans) = (((ans)) < ((rem)) ? ((ans)) : ((rem)));
  for (int(i) = 0; (i) < (p); ++(i)) {
    if (((mask) >> (i)&1)) continue;
    Dfs(((mask) | 1 << (i)), rem - cnt[i]);
  }
}
int main() {
  FILEIO();
  Read(n, p);
  for (int(i) = 1; (i) <= (n); ++(i)) {
    char c;
    In.Readalpha(c);
    a[i] = c - 'a';
    ++cnt[a[i]];
  }
  for (int(i) = 0; (i) < (p); ++(i))
    for (int(j) = 0; (j) < (p); ++(j)) Read(can[i][j]);
  for (int(i) = 0; (i) < (p); ++(i))
    for (int(j) = 0; (j) < (p); ++(j)) {
      if (can[i][j]) continue;
      int lastj = 0;
      int midmask = 0;
      for (int(k) = (n); (k) >= 1; --(k)) {
        if (a[k] == i) {
          if (not lastj) {
            if (a[k] == j) {
              lastj = k;
              midmask = 0;
            }
            continue;
          }
          Addban(midmask, i, j);
          lastj = 0;
          midmask = 0;
          if (a[k] == j) {
            lastj = k;
            midmask = 0;
          }
        } else if (a[k] == j) {
          lastj = k;
          midmask = 0;
        } else {
          midmask = ((midmask) | 1 << (a[k]));
        }
      }
    }
  ans = n;
  Dfs(0, n);
  cout << ans << endl;
  END();
}
