#include<bits/stdc++.h>
#define int long long
using namespace std;
namespace Testify{
    inline int read(){
        int f(1),x(0);
        char ch=getchar();
        for(;!isdigit(ch);ch=getchar()) if(ch=='-') f=-1;
        for(;isdigit(ch);ch=getchar()) x=(x<<1)+(x<<3)+(ch^48);
        return f*x;
    }
    inline void WritE(int x){
        if(x<0) putchar('-'),x=-x;
        if(x>9) WritE(x/10);
        putchar(x%10+48);
    }
    inline void write(int x){
        WritE(x);
        puts("");
    }
    inline void Write(int x){
        WritE(x);
        putchar(' ');
    }
}
using namespace Testify;
int n;
const int N=55;
int x,y,now[N],ans,Ans[N];
bool E[N][N];
mt19937 rd(chrono::system_clock::now().time_since_epoch().count());
inline int radom(int l,int r){
    return rd()%(r-l+1)+l;
}
inline int energy(){
    for(register int i=1;i<=n;i++){
        for(register int j=1;j<i;j++){
            if(!E[now[i]][now[j]]){
                return i-1;
            }
        }
    }
    return n;
}
inline void SA(){
    double T=3000;
    while(T>1e-15){
        int ra=radom(1,n),rb=radom(1,n);
        if(ra==rb){
            continue;
        }
        swap(now[ra],now[rb]);
        int e=energy();
        double delta=e-ans;
        if(delta>0){
            ans=e;
        }
        else if(exp(delta/T)*RAND_MAX<radom(0,RAND_MAX)){
            swap(now[ra],now[rb]);
        }
        T*=0.999;
    }
}
signed main(void){
    n=read();
    for(register int i=1;i<=n;i++){
        x=read(),y=read();
        now[i]=i;
        E[x][y]=E[y][x]=true;
    }
    ans=energy();
    while((double)clock()/CLOCKS_PER_SEC<0.99){
        SA();
    }
    write(ans);
    return 0;
}

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