#include <cstdio>

const int maxn = 30;

FILE *in = fopen("litere.in","r"), *out = fopen("litere.out","w");

int n;
int a[maxn];

int sol;

int main()
{
    fscanf(in, "%d", &n);

    char c;
    for ( int i = 1; i <= n; ++i )
    {
        fscanf(in, "%1s", &c);

        ++a[ c - 'a' ];

        for ( char j = 'a'; j <= 'z'; ++j )
            if ( j > c )
                sol += a[ j - 'a' ];
    }

    fprintf(out, "%d\n", sol);


	return 0;
}