#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
using namespace __gnu_pbds;
using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 1000LL * 1000LL * 1000LL * 1000LL * 1000LL * 1000LL;
const ll M = 1000LL * 1000LL * 1000LL + 7LL;
const int N = 1000 * 1000 + 7;
string tab[N];
ll il[N];

void Solve()
{
    int n;
    ll ans = 0LL, cur = 0LL;
    cin >> ans >> n; ans = 0LL;
    for(int i = 1; i <= n; ++i)
        cin >> tab[i] >> il[i];
    for(int i = n; i >= 1; --i)
    {
        ll s = 0LL, mi = 0LL;
        reverse(tab[i].begin(), tab[i].end());
        for(int j = 0; j < (int)tab[i].size(); ++j)
        {
            if(tab[i][j] == 'M')
                --s;
            else
                ++s;
            mi = min(mi, s); 
        }
        ans = min(ans, cur + mi);
        ans = min(ans, cur + s * (il[i] - 1LL) + mi);
        cur += il[i] * s;
    }
    ans = max(0LL, -ans - 1LL);
    if(cur < 0)
        cout << -1 << "\n";
    else
        cout << ans << "\n";
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    //int t; cin >> t;
    //while(t--)
        Solve();

    return 0;
}