#include <bits/stdc++.h>
using namespace std;
#define ll long long
const ll nx=1e5+5;
ll n, m, cnt[nx], sm, mnpref;
string s[nx];
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n>>m;
for (int i=1; i<=m; i++) cin>>s[i]>>cnt[i];
for (int i=m; i>=1; i--)
{
ll sz=s[i].size(), cursm=0, curmn=0;
for (int j=sz-1; j>=0; j--)
{
if (s[i][j]=='F') cursm++;
else cursm--;
curmn=min(curmn, cursm);
}
mnpref=min({mnpref, sm+curmn, sm+(cnt[i]-1)*cursm+curmn});
sm+=cursm*cnt[i];
//cout<<"debug "<<sm<<' '<<mnpref<<'\n';
}
if (sm<0) cout<<-1;
else cout<<max(0ll, -1-mnpref);
}
