#include <cassert>
#include <cstdio>

const int d=30;
int v[d];

int main()
{
    int n=0,i=0,j=0,sol=0,nr=0;
    char c=' ';

    assert(freopen("litere.in","r",stdin));
    assert(freopen("litere.out","w",stdout));

    assert(scanf("%d\n",&n));
    for(i=0; i<n; ++i)
    {
        assert(scanf("%c",&c));
        nr=c-'a'+2;
        ++v[nr-1];

        while (nr<27)
        {
            sol+=v[nr];
            ++nr;
        }
    }
    assert(printf("%d\n",sol));
    return 0;
}