#include <cstdio>
#include <algorithm>
using namespace std;
const int MAX = 400020;
int circle, n;
bool data[MAX<<1];
void input(){
scanf("%d%d", &circle, &n);
n <<= 1;
int i;
for(i = 0; i<n; i++){
char t[2];
scanf("%*d%s", t);
data[i+n] = data[i] = t[0] == 'W';
}
}
int fail[MAX];
void calcFail(){
fail[0] = -1;
int i;
for(i = 1; i<n; i++){
int last = i-1;
fail[i] = -1;
while(last >= 0){
if(data[i] == data[fail[last]+1]) {
fail[i] = fail[last]+1;
break;
} else last = fail[last];
}
}
}
int res = 0;
void check(int pos){
int t = min(pos, n-pos);
if(res == 0 || res > t) res = t;
}
void match(){
int f = 0, s;
for(s = 0; s<2*n; s++){
while(f > 0 && data[f] == data[s])
f = fail[f-1]+1;
if(data[f] != data[s]) f++;
if(f == n){
check(s-(n-1));
f = fail[n-1]+1;
}
}
}
int main(){
input();
calcFail();
match();
printf("%lld\n", res == 0 ? -1 : (long long)res * n);
return 0;
}
