KB C++

#include <bits/stdc++.h>

using namespace std;

int n,a[100000];
string s;

void ReadData()
{
    cin>>n;
    cin>>s;
}

void solve()
{
    if(n>26)
    {
        cout<<-1;
        return;
    }
    for(int i=0;i<n;++i)
        a[int(s[i])++;
    int cnt=0;
    for(int i=97;i<123;++i)
        if(a[i]>1)
            cnt=cnt+(a[i]-1);
    cout<<cnt;
}

int main()
{
    freopen("kb.inp", "r", stdin);
    freopen("kb.out", "w", stdout);
    ReadData();
    solve();
    return 0;
}