#include <iostream>

 #include <cmath>

 #include <cstdio>

 #include <cstring>

 #include <string>

 #include <map>

 #include <iomanip>

 #include <algorithm>

 #include <queue>

 #include <stack>

 #include <set>

 #include <vector>

 //const int maxn = 1e5+5;

 #define ll long long

 //#define MAX INT_MAX

 #define FOR(i,a,b) for( int i = a;i <= b;++i)

 //#define MOD 142857

 using namespace std;

 char c;

 int n,minn=INT_MAX,ans1,ans2,ans3,ans4,sum,a,b,ans;

 string s;

 int main()

 {

 //    freopen("D:\\common_text\\code_stream\\in.txt","r",stdin);

 //    freopen("D:\\common_text\\code_stream\\out.txt","w",stdout);

     cin>>n>>s;

     for(int i=0;i<=n-4;++i)

     {

         sum=;

         int a=abs(s[i]-'A');

         int b=abs(s[i+]-'C');

         int c=abs(s[i+]-'T');

         int d=abs(s[i+]-'G');

         sum+=min(a,-a)+min(b,-b)+min(c,-c)+min(d,-d);

         if(minn>sum)

             minn=sum;

     }

     cout<<minn;

 }