#include<bits/stdc++.h>
#define int long long
#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 fi first
#define se second
#define eb emplace_back
#define popc __builtin_popcount
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pii> vp;
typedef unsigned long long ull;
typedef long double ld;

int read() {
	int x=0,w=1; char c=getchar(); 
	while(!isdigit(c)) {if(c=='-') w=-1; c=getchar();}
	while(isdigit(c)) {x=x*10+c-'0'; c=getchar();}
	return x*w;
}

const int N=2e5+9;
int n,m,c[N],ans,f[N],l[N],r[N];
char s[N];

signed main() {
	n=read(), m=read();
	rep(i,1,m) {
		l[i]=r[i-1]+1; scanf("%s%lld",s+l[i],&c[i]); 
		int len=strlen(s+l[i]); r[i]=l[i]+len-1;
	}
	per(i,m,1) {
		int cnt=0;
		rep(j,l[i],r[i]) cnt+=(s[j]=='M'?1:-1);
		if(cnt>0) f[r[i]+1]+=cnt*(c[i]-1);
		per(j,r[i],l[i]) {
			f[j]=(f[j+1]+(s[j]=='M'?1:-1));
			if(s[j]=='F') ans=max(ans,f[j]);
		}
		if(cnt<0) f[l[i]]+=cnt*(c[i]-1);
	}
	if(f[1]>0) puts("-1");
	else printf("%lld\n",ans);
	return 0;
}