#import<bits/stdc++.h>
std::ifstream f("litere.in");std::ofstream g("litere.out");int n,v[124],i,j;std::string s;main(){f>>n>>s;for(i=0;i<n;++i){v[s[i]]++;for(j=122;j>s[i];j--)v[0]+=v[j];}g<<v[0];}