#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=(a);i<=(b);++i)
#define per(i,a,b) for(int i=(a);i>=(b);--i)
#define pii pair<int,int>
#define fi first
#define se second
#define pb push_back
#define ll long long
using namespace std;
inline ll read(){
    ll x=0,f=1;char ch=getchar();
    while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}
    while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
    return x*f;
}
const int N=2e5+5;
ll n,m,k,cur,Mx;
char s[N];
int main(){
    n=read(),m=read();
    while(m--){
        scanf("%s",s+1);
        k=read();
        int len=strlen(s+1);
        int mx=0,cnt=0;
        rep(i,1,len){
            if(s[i]=='F')cnt++;
            else --cnt;
            mx=max(mx,cnt);
        }
        if(cnt>=0)Mx=max(Mx,cur+mx+1ll*cnt*(k-1));
        else Mx=max(Mx,cur+mx);
        cur+=cnt*k;
    }
    if(cur<0)return puts("-1"),0;
    printf("%lld\n",max(0ll,Mx-cur-1));
    return 0;
}