#include <bits/stdc++.h>
using namespace std;
#define int long long
#define sz(x) x.size()
#define pb push_back
#define F first
#define S second
#define nl '\n'
void solve()
{
int n;
string s;
cin >>n >>s;
int cntr = 0, cntg = 0;
vector< int >r, g;
int pr[n], pg[n];
pr[0] = pg[0] = 0;
for( int i = 0; i < n; ++i )
{
if( i > 0 )
{
pr[i] = pr[i - 1];
pg[i] = pg[i - 1];
}
if( s[i] == 'R' )
{
r.pb( i );
++pr[i];
++cntr;
}
else
{
g.pb( i );
++pg[i];
++cntg;
}
}
if( cntg > cntr )
{
swap(cntr, cntg);
swap(r, g);
for( int i = 0; i < n; ++i )
swap(pr[i], pg[i]);
}
if( cntr - cntg > 1 )
{
cout <<-1;
return;
}
int ans = 0;
if( cntr > cntg )
{
int pos = 1;
for( int i: g )
{
ans += abs(pos - i);
pos += 2;
}
}
else
{
int cnt1 = 0, cnt2 = 0, pos = 0;
for( int i: r )
{
cnt1 += abs(i - pos);
pos += 2;
}
pos = 0;
for( int i: g )
{
cnt2 += abs(i - pos);
pos += 2;
}
ans = min(cnt1, cnt2);
}
cout <<ans;
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
// freopen("input.txt", "r", stdin);
// freopen("output.txt", "w", stdout);
int T = 1;
// cin >>T;
while( T-- )
solve();
}
