#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int n, m;

int main()
{
  ios::sync_with_stdio(0), cin.tie(0);
  cin >> n >> m;
  ll minn = -1, dif = 0;
  for (int c, i = 0; i < m; i++)
  {
    string s;
    cin >> s >> c;

    ll subdif = 0, submin = 0;
    for (auto z : s)
    {
      subdif += (z == 'M' ? 1LL : -1LL);
      submin = min(submin, subdif);
    }
    ll totmin = 0;
    if (subdif < 0)
    {
      totmin = subdif * (c - 1);
    }
    totmin += submin;
    minn = min(minn, dif + totmin);

    dif += subdif * c;
  }

  if (dif > 0)
  {
    cout << "źle1\n";
    cout << "-1\n";
    return 0;
  }

  cout << max(0LL, dif - (minn + 1)) << "\n";
}

/*
2 1
FFMF 1

6 1
MFFFMFMMFFFM 1

6 1
FFFMMMMMMFFF 1

6 3
F 3
M 6
F 3
*/