#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define int long long
#define vi vector<int>
#define vvi vector<vector<int>>
#define pii pair<int, int>
#define vpii vector<pair<int, int>>
#define vc vector<char>
#define vb vector<bool>
#define mii map<int,int>
#define f0r(i,n) for(int i=0;i<n;i++)
#define FOR(i,k,n) for(int i=k;i<n;i++)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define in(a) int a; cin>>a
#define in2(a,b) int a,b; cin>>a>>b
#define in3(a,b,c) int a,b,c; cin>>a>>b>>c
#define in4(a,b,c,d) int a,b,c,d; cin>>a>>b>>c>>d
#define vin(v,n); vi v(n); f0r(i,n){cin>>v[i];}
#define out(a) cout<<a<<'\n'
#define out2(a,b) cout<<a<<' '<<b<<'\n'
#define out3(a,b,c) cout<<a<<' '<<b<<' '<<c<<'\n'
#define out4(a,b,c,d) cout<<a<<' '<<b<<' '<<c<<' '<<d<<'\n'
#define pout(a) cout<<a.first<<' '<<a.second<<'\n'
#define vout(v) for(auto u : v){cout<<u<<' ';} cout<<'\n'
#define dout(a) cout<<a<<' '<<#a<<'\n'
#define dout2(a,b) cout<<a<<' '<<#a<<' '<<b<<' '<<#b<<'\n'
#define yn(x); if(x){cout<<"YES"<<'\n';}else{cout<<"NO"<<'\n';}
const int leg = 1e9 + 7;
const int mod = 998244353;
using namespace std;
signed main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
//ifstream cin(".in");
//ofstream cout(".out");
in2(n,m);
vector<pair<string,int>>v;
f0r(i,m){
string s;
int x;
cin>>s>>x;
v.pb(mp(s,x));
}
int cur = 0;
int mn = 0;
int ms = 0;
int fs = 0;
for(int i = m-1; i>=0; i--){
int run = 0;
int mnrun = 0;
string s = v[i].first;
int fc = 0;
int mc = 0;
for(int j = s.size() - 1; j>=0; j--){
if(s[j] == 'F'){
run++;
fc++;
}
else{
run--;
mc++;
}
mnrun = min(mnrun, run);
}
ms += mc * v[i].second;
fs += fc * v[i].second;
f0r(j, v[i].second){
mn = min(mn, j * run + cur + mnrun);
}
cur += v[i].second * run;
// dout(cur);
}
// dout(mn);
if(ms > fs){
out(-1);
}
else if(mn >= -1)out(0);
else out(-mn - 1);
}
