Limak

#include <bits/stdc++.h>
using namespace std;
#define PB push_back
#define ZERO (1e-10)
#define INF (1<<29)
#define CL(A,I) (memset(A,I,sizeof(A)))
#define DEB printf("DEB!\n");
#define D(X) cout<<"  "<<#X": "<<X<<endl;
#define EQ(A,B) (A+ZERO>B&&A-ZERO<B)
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;
#define IN(n) int n;scanf("%d",&n);
#define FOR(i, m, n) for (int i(m); i < n; i++)
#define REP(i, n) FOR(i, 0, n)
#define F(n) REP(i, n)
#define FF(n) REP(j, n)
#define FT(m, n) FOR(k, m, n)
#define aa first
#define bb second
void ga(int N,int *A){F(N)scanf("%d",A+i);}
#define MX (1<<19)
#define IX(C) (C=='A'?0:C=='C'?1:C=='G'?2:3)
char s[MX];
int N,A[MX][4],q[4],O;
void gt(int b,int e,int*q){
    F(4)q[i]=A[N][i];
    F(4)q[i]-=A[e][i];
    F(4)q[i]+=A[b-1][i];
}
bool ok(int d){
    F(N-d+1){
        int S(0);
        gt(i+1,i+d,q);
        F(4)S+=abs(O-q[i]);
        if(S<=d)return 1;
    }
    return 0;
}
int main(void){
    scanf("%d%s",&N,s+1);
    FT(1,N+1)memcpy(A[k],A[k-1],sizeof(A[k])),++A[k][IX(s[k])];
    O=N>>2;
    int B(0),E(N),M;
    gt(1,5,q);
    while(B+1<E){
        if(!ok(M=(B+E+1)/2))B=M+1;
        else E=M;
    }
    printf("%d\n",B+!ok(B));
    return 0;
}