#include <bits/stdc++.h>
using namespace std;

typedef unsigned uint;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii> vpii;
typedef complex<double> cmplx;
template <typename T> using minPq = priority_queue<T, vector<T>, greater<T>>;
#define boost() cin.sync_with_stdio(0); cin.tie(0)
#define ms(a, x) memset(a, x, sizeof(a))
#define pb push_back
#define fs first
#define sn second
#define ALL(v) (v).begin(), (v).end()
#define SZ(v) ((int) (v).size())
#define lbv(v, x) (lower_bound((v).begin(), (v).end(), x) - (v).begin())
#define ubv(v, x) (upper_bound((v).begin(), (v).end(), x) - (v).begin())
template <typename T> inline void UNIQUE(vector<T> &v) {
    sort(ALL(v));
    v.resize(unique(ALL(v)) - v.begin());
}
const int INF = 0x3f3f3f3f;
const ll LLINF = 0x3f3f3f3f3f3f3f3f;
const double PI = acos(-1);
#define FR(i, n) for(int i = 0; i < (n); i++)
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define FORR(i, a, b) for(int i = (a); i >= (b); i--)
#define dbg(x) {cerr << #x << ' ' << x << endl;}
#define dbgArr(arr, n) {cerr << #arr; FR(_i, n) cerr << ' ' << arr[_i]; cerr << endl;}

ll n, m;
ll sum, mn;

int main() {
    boost();
    cin >> n >> m;

    while (m--) {
        string str;
        ll freq;
        cin >> str >> freq;

        int tmpSum = 0, tmpMn = 0;

        for (char ch : str) {
            int v = ch == 'M' ? 1 : -1;
            tmpSum += v;
            tmpMn = min(tmpMn, tmpSum);
        }

        mn = min(mn, sum + tmpMn);
        mn = min(mn, sum + (ll) tmpSum * (freq - 1) + tmpMn);
        sum += (ll) tmpSum * freq;
    }

    if (sum > 0)
        return !printf("-1\n");

    assert(sum % 2 == 0);

    cout << max(0LL, sum - mn - 1) << '\n';
}