#include <bits/stdc++.h>
using namespace std;
#define l2 long long
int n;
l2 ft[123456];
void up(int x, l2 k) {
while (x <= (l2)n) {
ft[x] += k;
x += (x & (-x));
}
}
l2 q(int x) {
l2 ans = 0;
while (x > 0) {
ans += ft[x];
x -= (x & (-x));
}
return ans;
}
int main() {
cin >> n;
string s;
cin >> s;
queue<int> v[26];
for (int i = 0; i < n; i++) v[s[n+i]-97].push(i+1);
l2 ans = 0;
memset(ft, 0, sizeof(ft));
for (int i = 1; i <= n; i++) up(i, 1);
for (int i = 0; i < n; i++) {
ans += q(v[s[i]-97].front()-1);
up(v[s[i]-97].front(), -1);
v[s[i]-97].pop();
}
cout << ans;
return 0;
}
