import std.algorithm, std.conv, std.range, std.stdio, std.string;

void main()
{
  auto n = readln.chomp.to!size_t;
  auto s = readln.chomp;

  // abcdefghijklmnopqrstuvwxyz
  // 01234567890123456789012345
  auto ca = 0, ci = 8, ck = 10, cu = 20, cy = 24, cz = 25;

  auto hi = new int[](26);
  foreach (c; s) ++hi[c - 'a'];

  auto r = 0;

  auto rz = hi[cz];
  hi[cz] -= rz;
  r += rz;

  auto ry = hi[cy];
  hi[cy] -= ry;

  foreach (c; cu+1..cz+1) {
    auto ry_ = min(ry, hi[c]);
    hi[c] -= ry_;
    ry -= ry_;
    r += ry_;
  }

  auto ryu = min(ry, hi[cu]);
  hi[cu] -= ryu;

  foreach (c; ck+1..cz+1) {
    auto ryu_ = min(ryu, hi[c]);
    hi[c] -= ryu_;
    ryu -= ryu_;
    r += ryu_;
  }

  auto ryuk = min(ryu, hi[ck]);
  hi[ck] -= ryuk;

  foreach (c; ci+1..cz+1) {
    auto ryuk_ = min(ryuk, hi[c]);
    hi[c] -= ryuk_;
    ryuk -= ryuk_;
    r += ryuk_;
  }

  auto ryuki = min(ryuk, hi[ci]);
  hi[ci] -= ryuki;

  foreach (c; ca..cz+1) {
    auto ryuki_ = min(ryuki, hi[c]);
    hi[c] -= ryuki_;
    ryuki -= ryuki_;
    r += ryuki_;
  }

  writeln(r);
}