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

string S;
vector<long> P;
int main()
{
  ios::sync_with_stdio(0), cin.tie(0);
  cin >> n >> m;
  for (int c, i = 0; i < m; i++)
  {
    string x;
    cin >> x >> c;
    for (; c--;)
      S.append(x);
  }
  P.resize(2 * n + 1);
  long dif = 0;
  long minn = 0;
  for (int i = 0; i < 2 * n; i++)
  {
    dif += S[i] == 'M' ?: -1;
    P[i + 1] = P[i] + (S[i] == 'M' ?: -1);
    minn = min(minn, P[i + 1]);
  }
  // for (int i = 0; i < 2 * n; i++)
  // {
  //   cout << P[i + 1] << " ";
  // }
  // cout << "\n";
  // cout << dif << " " << minn << "\n";
  if (dif > 0 || P[2 * n] < dif)
  {
    cout << "-1\n";
    return 0;
  }
  
  cout << dif - (minn + 1) << "\n";
}

/*
6
1
MFFFMFMMFFFM 1

6
1
FFFMMMMMMFFF 1

6
3
F 3
M 6
F 3
*/