A

#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n;
    cin>>n;

    string s;
    cin>>s;

    for(int i=0; i+2<n; i++)
    {
        if(s[i]=='A' && s[i+1]=='B' && s[i+2]=='C')
            return cout<<i+1<<"\n", 0;
    }

    cout<<-1<<"\n";
}