#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    string s;
    cin >> n >> s;
    bool v[4] = {0}, bk = 0;
    for(int i = 0; i < n; i++){
        if(s[i] == 'A'){ 
            v[1] = 1;
            if(s[i+1] == 'B'){ 
                v[2] = 1;
                if(s[i+2] == 'C'){ 
                    v[3] = 1;
                    if(v[1] == 1 && v[2] == 1 && v[3] == 1){
                        cout << i + 1;
                        bk = 1;
                        break;
                    }
                }    
            }
        
        }          
    }
    if(bk == 0) cout << "-1";
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main()
{
	int n;cin>>n;
	string s;cin>>s;
	for(int i=0;i<=n-3;i++)
	{
		if(s[i]=='A'&&s[i+1]=='B'&&s[i+2]=='C')
		{
			cout<<i+1;
			return 0;
		}
	}
	cout<<-1;
	return 0;
}