#include<stdio.h>

const int NMAX = 1e4 + 5;

char s[NMAX];
int f[300];

int main() {
    freopen("litere.in", "r", stdin);
    freopen("litere.out", "w", stdout);
    int n, i, lit, inv;

    scanf("%d\n%s", &n, &s);
 
    inv = 0;
    for(i = 0; i < n; ++ i) {
        for(lit = s[i] + 1; lit <= 'z'; ++ lit)
            inv += f[lit];

        ++ f[s[i]];
    }

    printf("%d\n", inv);
    return 0;
}