#include <bits/stdc++.h>
using namespace std;
#define st first
#define nd second
#define pb push_back
typedef long long ll;
vector<pair<string, ll>> parts;
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	ll n, mc=0, lic=0;
	ll m, x, out=0;
	cin >> n >> m;
	for(int i=0; i<m; i++){
		string s;
		cin >> s >> x;
		parts.pb({s, x});
	}
	for(int i=m-1; i>=0; i--){
		int ln=parts[i].st.size();
		ll tmpMc=0, mn=0, tmp=0;
		
		for(int j=ln-1; j>=0; j--){
			if(parts[i].st[j]=='M'){
				tmpMc++;
				tmp--;
			}else tmp++;
			mn=min(mn, tmp);
		}
		
		tmpMc*=parts[i].nd;
		out=min(out, lic+mn);
		out=min(out, lic+(parts[i].nd-1)*tmp+mn);
		lic+=parts[i].nd*tmp;
		mc+=tmpMc;
	}
	if(mc>n){
		cout << -1 << '\n';
		return 0;
	}
	out*=(-1);
	out--;
	cout << max((ll)0, out) << '\n';
	return 0;
}