#include<bits/stdc++.h>
#define re register
using namespace std;
inline int read(){
	re int t=0;re char v=getchar();
	while(v<'0')v=getchar();
	while(v>='0')t=(t<<3)+(t<<1)+v-48,v=getchar();
	return t;
}
const int M=998244353;
inline void add(re int &x,re int y){(x+=y)>=M?x-=M:x;}
inline int Mod(re int x){return x>=M?x-M:x;}
inline int ksm(re int x,re int y){
	re int s=1;
	while(y){
		if(y&1)s=1ll*s*x%M;
		x=1ll*x*x%M,y>>=1;
	}
	return s;
}
int t,n,m,a[1000002],f[302][302][302],ans,A,B,R[1000002],stk[1000002],tp,num[2],sum[302][2];
char s[1000002];
int main(){
	n=read();
	scanf("%s",s+1);
	for(re int i=1;i<=n;++i)s[i]%=2,++num[s[i]];
	if(n&1){
		for(re int i=1;i<=n;++i)if(num[s[i]]&1)f[i][i][0]=1;
	}
	else{
		for(re int i=1;i<n;++i)f[i+1][i][0]=0;
	}
	for(re int i=1;i<=n;++i)sum[i][0]=sum[i-1][0],sum[i][1]=sum[i-1][1],++sum[i][s[i]];
	if(n%2==0&&num[0]%2==1)return puts("-1"),0;
	if(num[0]&1)--num[0];if(num[1]&1)--num[1];
	for(re int l=n;l;--l)
		for(re int r=l+1;r<=n;++r)
			for(re int k=0;k<=r-l+1;++k){
				f[l][r][k]=max(f[l][r-1][k],f[l+1][r][k]);
				if(s[l]==s[r]&&k>=s[l]&&(n%2==0||f[l+1][r-1][k-s[l]]%2==1))f[l][r][k]=max(f[l][r][k],f[l+1][r-1][k-s[l]]+2);
			}
	ans=n>>1;
	for(re int l=1;l<=n;++l)
		for(re int r=l;r<=n;++r)
			for(re int k=0;k<=n;++k)if(f[l][r][k]){
				int dlt1=num[1]-k*2,dlt0=num[0]-(f[l][r][k]-k*2-f[l][r][k]%2);
				if(sum[l-1][1]>=(dlt1>>1)&&sum[l-1][0]>=(dlt0>>1))ans=max(ans,f[l][r][k]+min(dlt1>>1,sum[l-1][1])+min(dlt0>>1,sum[l-1][0]));
			}
	printf("%d",n-ans);
}