#include <bits/stdc++.h>
using namespace std;
#define fileio() freopen("input.txt","r",stdin); freopen("output.txt","w",stdout)
#define fio() ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define all(x) (x).begin(), (x).end()
#define allr(x) x.rbegin(), x.rend()
#define cmprs(x) sort(all(x)),x.erase(unique(all(x)),x.end())
#define endl "\n"
#define sp " "
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define F first
#define S second
#define rz resize
#define sz(a) (int)(a.size())
#define clr(a) a.clear()
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<int, ll> pil;
typedef tuple<int, int, int> tpi;
typedef tuple<ll, ll, ll> tpl;
typedef pair<double, ll> pdl;
typedef pair<double, int> pdi;
const int dx[] = {1,-1,0,0,1,1,-1,-1};
const int dy[] = {0,0,1,-1,1,-1,1,-1};
const ll MOD = 1e9+7;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3f;
const int MAX = 101010; // PLZ CHK!
ll N,M,K[MAX];
string A[MAX];
int main() {
fio();
cin>>N>>M;
for (int i=0; i<M; i++) cin>>A[i]>>K[i];
ll am=0,af=0,ans=0;
for (int i=M-1; i>=0; i--) {
ll m=0,f=0,mx=0;
string s=A[i];
for (int j=sz(s)-1; j>=0; j--) {
if (s[j]=='M') m++;
else f++;
if (m>=f+2) {
ans=max(ans, am+m-af-f-1);
}
}
am+=(K[i]-1)*m,af+=(K[i]-1)*f;
ll tm=0,tf=0;
for (int j=sz(s)-1; j>=0; j--) {
if (s[j]=='M') tm++;
else tf++;
ans=max(ans, am+tm-af-tf-1);
}
am+=m, af+=f;
}
if (am>af) {
cout<<-1;
}
else cout<<ans;
return 0;
}
