#include <bits/stdc++.h>
using namespace std;
const long long maxn = 1e5 + 5;
const long long maxm = 1e3 + 5;
namespace IO {
const int SIZE = (1 << 20) + 1;
char ibuf[SIZE], *iS, *iT, obuf[SIZE], *oS = obuf, *oT = obuf + SIZE - 1;
char _st[55];
int _qr = 0;
inline char gc() {
  return (iS == iT ? iT = (iS = ibuf) + fread(ibuf, 1, SIZE, stdin),
          (iS == iT ? EOF : *iS++) : *iS++);
}
inline void qread() {}
template <class T1, class... T2>
inline void qread(T1 &IEE, T2 &...ls) {
  register T1 __ = 0, ___ = 1;
  register char ch;
  while (!isdigit(ch = gc())) ___ = (ch == '-') ? -___ : ___;
  do {
    __ = (__ << 1) + (__ << 3) + (ch ^ 48);
  } while (isdigit(ch = gc()));
  __ *= ___;
  IEE = __;
  qread(ls...);
  return;
}
inline void flush() {
  fwrite(obuf, 1, oS - obuf, stdout);
  oS = obuf;
  return;
}
inline void putc_(char _x) {
  *oS++ = _x;
  if (oS == oT) flush();
}
inline void qwrite() {}
template <class T1, class... T2>
inline void qwrite(T1 IEE, T2... ls) {
  if (!IEE) putc_('0');
  if (IEE < 0) putc_('-'), IEE = -IEE;
  while (IEE) _st[++_qr] = IEE % 10 + '0', IEE /= 10;
  while (_qr) putc_(_st[_qr--]);
  qwrite(ls...);
  return;
}
struct Flusher_ {
  ~Flusher_() { flush(); }
} io_flusher;
}  // namespace IO
using namespace IO;
long long n, p;
char s[maxn];
long long cnt[maxn][21];
bool sta[17][17][1 << 17];
long long las[maxn];
bool f[maxn];
long long g[21][21];
long long mlg[maxn];
inline long long low(long long x) { return x & (-x); }
int main() {
  scanf("%lld %lld", &n, &p);
  scanf("%s", s + 1);
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < p; j++) {
      if (las[j] == 0) continue;
      long long fl = 0;
      for (int k = 0; k < p; k++) {
        if (cnt[i - 1][k] - cnt[las[j]][k] == 0) continue;
        fl |= (1 << k);
      }
      sta[s[i] - 'a'][j][fl] = 1;
    }
    for (int j = 0; j < p; j++) {
      cnt[i][j] = cnt[i - 1][j] + (s[i] - 'a' == j);
    }
    las[s[i] - 'a'] = i;
  }
  for (int i = 0; i < p; i++) mlg[1 << i] = i;
  for (int i = 0; i < p; i++) {
    for (int j = 0; j < p; j++) {
      for (int k = 0; k < (1 << p); k++) {
        for (int t = k; t; t -= low(t)) {
          sta[i][j][k] |= sta[i][j][k ^ low(t)];
        }
      }
    }
  }
  for (int i = 0; i < p; i++) {
    for (int j = 0; j < p; j++) {
      scanf("%lld", &g[i][j]);
    }
  }
  long long ans = n, sum = 0;
  f[0] = 1;
  for (int i = 0; i < (1 << p); i++) {
    if (i == 0) continue;
    f[i] = 1;
    for (int j = 0; j < p; j++) {
      if ((i >> j) & 1) continue;
      for (int k = 0; k < p; k++) {
        if ((i >> k) & 1) continue;
        if (g[j][k] == 1) continue;
        if (sta[j][k][i]) {
          f[i] = 0;
          break;
        }
      }
    }
    bool flag = 0;
    for (int j = 0; j < p; j++) {
      if ((i >> j) & 1) {
        flag |= f[i ^ (1 << j)];
      }
    }
    f[i] &= flag;
    sum = 0;
    for (int j = 0; j < p; j++) {
      if (((i >> j) & 1) == 0) sum += cnt[n][j];
    }
    if (f[i]) ans = min(ans, sum);
  }
  cout << ans << endl;
  return 0;
}
