输入一个字符串，求一个最长的子串，使得它是原字符串的前缀，后缀，和一个不是前缀和后缀的子串。

z-algorithm求出z数组，然后贪心匹配。

#include<bits/stdc++.h> 
using namespace std;
#define For(i,n) for(int i=1;i<=n;i++)
#define Fork(i,k,n) for(int i=k;i<=n;i++)
#define ForkD(i,k,n) for(int i=n;i>=k;i--)
#define Rep(i,n) for(int i=0;i<n;i++)
#define ForD(i,n) for(int i=n;i;i--)
#define RepD(i,n) for(int i=n;i>=0;i--)
#define Forp(x) for(int p=pre[x];p;p=next[p])
#define Forpiter(x) for(int &p=iter[x];p;p=next[p])  
#define Lson (o<<1)
#define Rson ((o<<1)+1)
#define MEM(a) memset(a,0,sizeof(a));
#define MEMI(a) memset(a,0x3f,sizeof(a));
#define MEMi(a) memset(a,128,sizeof(a));
#define MEMx(a,b) memset(a,b,sizeof(a));
#define INF (0x3f3f3f3f)
#define F (1000000007)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define vi vector<int> 
#define pi pair<int,int>
#define SI(a) ((a).size())
#define Pr(kcase,ans) printf("Case #%d: %lld\n",kcase,ans);
#define PRi(a,n) For(i,n-1) cout<<a[i]<<' '; cout<<a[n]<<endl;
#define PRi2D(a,n,m) For(i,n) { \
						For(j,m-1) cout<<a[i][j]<<' ';\
						cout<<a[i][m]<<endl; \
						} 
#pragma comment(linker, "/STACK:102400000,102400000")
#define ALL(x) (x).begin(),(x).end()
#define gmax(a,b) a=max(a,b);
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
ll mul(ll a,ll b){return (a*b)%F;}
ll add(ll a,ll b){return (a+b)%F;}
ll sub(ll a,ll b){return ((a-b)%F+F)%F;}
void upd(ll &a,ll b){a=(a%F+b%F)%F;}
inline int read()
{
	int x=0,f=1; char ch=getchar();
	while(!isdigit(ch)) {if (ch=='-') f=-1; ch=getchar();}
	while(isdigit(ch)) { x=x*10+ch-'0'; ch=getchar();}
	return x*f;
} 
void z_function(string s, vector<int> &z) {
  int n = s.size();
  z.assign(n, 0);
  int L = 0, R = 0;
  for(int i=1;i<n;i++) {
    if (i > R) {
      L = R = i;
      while (R < n && s[R-L] == s[R]) R++;
      z[i] = R-L; R--;
    } else {
      int k = i-L;
      if (z[k] < R-i+1) z[i] = z[k];
      else {
        L = i;
        while (R < n && s[R-L] == s[R]) R++;
        z[i] = R-L; R--;
      }
    }
  }
}
int main()
{
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
	string s;cin>>s;
	vi z;
	z_function(s,z);
	int n=s.length(),an=-1,ma=0;
	For(i,n-1) {
		if(z[i]+i==n) {
			if(ma>=z[i]) gmax(an,z[i])
		}
		gmax(ma,z[i])
	}	
	if(an==-1)puts("Just a legend");
	else cout<<(s.substr(0,an));
	
	return 0;
}