#include <bits/stdc++.h>
using namespace std;
#ifdef MIKU
string dbmc = "\033[1;38;2;57;197;187m", dbrs = "\033[0m";
#define debug(x...) cout << dbmc << "[" << #x << "]: ", dout(x)
void dout() { cout << dbrs << endl; }
template <typename T, typename ...U>
void dout(T t, U ...u) { cout << t << (sizeof...(u) ? ", " : ""); dout(u...); }
#else
#define debug(...) 39
#endif
#define fs first
#define sc second
#define mp make_pair
#define FOR(i, j, k) for (int i = j, Z = k; i < Z; i++)
using ll = long long;
typedef pair<int, int> pii;
const int MXN = 100005;
ll n;
int m;
string s[MXN];
ll mul[MXN];
ll now = 0, ans = 0;
void DO(string &s, ll k) {
reverse(s.begin(), s.end());
ll d = 0, low = 0;
for (auto &i : s) {
if (i == 'F') d++;
else {
d--;
low = min(low, d);
}
}
low = min(low, d * (k - 1) + low);
d *= k;
ans = min(ans, now + low);
now += d;
}
void miku() {
cin >> n >> m;
FOR(i, 0, m) cin >> s[i] >> mul[i];
for (int i = m - 1; i >= 0; i--) {
DO(s[i], mul[i]);
}
if (now < 0) {
cout << -1 << '\n';
return;
}
cout << max(0LL, -ans - 1) << '\n';
}
int32_t main() {
cin.tie(0) -> sync_with_stdio(false);
cin.exceptions(cin.failbit);
miku();
return 0;
}
