#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
#define int long long
typedef pair<int,int> pii;
typedef vector<pii> vii;
typedef vector<int> vi;
typedef vector<bool> vb;
#define FOR(i,a) for(int i=0;i<(a);i++)
#define FORE(i,a,b) for(int i=(a);i<(b);i++)
#define all(x) x.begin(),x.end()
#define fi first
#define se second
#define pb push_back
#define sp <<" "<<
#define cont(x) for(auto el:x) cout<<el<<' ';cout<<endl;
#define contp(x) for(auto el:x) cout<<el.fi<<'-'<<el.se<<' ';cout<<endl;
#define DEBUG(x) cout<<#x sp x<<endl;
#define carp(x,y) ((x%MOD)*(y%MOD))%MOD
#define topla(x,y) ((x%MOD)+(y%MOD))%MOD
#define mid (l+r)/2
const int MAXN=2e3+5;
const int INF=1e18;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin>>n;
string s;
cin>>s;
//ya a'lar başta
//ya b'ler başta
vi bir;
vi iki;
FOR(i,s.length()){
int deg=s[i]-'a';
if(deg==0LL) bir.pb(i);
else iki.pb(i);
}
int res=0;
int res2=0;
int ind=0;
for(int i=0;i<2*n;i+=2){
res+=abs(bir[ind]-i);
res2+=abs(iki[ind]-i);
ind++;
}
cout<<min(res,res2)<<endl;
}
