#include <bits/stdc++.h>
#define int long long
using namespace std;
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0), cout.tie(0);
int N, M, K, sm = 0, cur = 0, mn = 0, smn;
vector<pair<string, int>> inp;
string S;
cin >> N >> M;
while (M--) {
cin >> S >> K;
inp.push_back({S, K});
}
reverse(inp.begin(), inp.end());
for (pair<string, int> i : inp) {
S = i.first, K = i.second, cur = 0, smn = 0;
for (int i = S.size() - 1; i >= 0; i--) {
cur += (S[i] == 'F' ? 1 : -1);
smn = min(smn, cur);
}
mn = min({mn, sm + smn, sm + cur * (K - 1) + smn});
sm += cur * K;
}
if (sm < 0) cout << -1;
else cout << max(0ll, -mn - 1);
}
