#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <iterator>
#include <complex>
#include <queue>
#include <stack>
#include <cmath>
#include <cassert>
using namespace std;
typedef long long LL;

class LikelyWord
{
public:
	int likely(vector <string> dictionary, int k) 
	{
		vector<LL> dl, dr;
		dl.push_back(-1);
		dr.push_back(-1);
		for(int i=0; i<dictionary.size(); ++i) {
			dl.push_back( toLLl(dictionary[i],k) );
			dr.push_back( toLLr(dictionary[i],k) );
		}
		dl.push_back( ipow(26,k) );
		dr.push_back( ipow(26,k) );

		LL max=-1, maxi=-1;
		for(int i=0; i+1<dl.size(); ++i) {
			LL hm = dr[i+1] - dl[i] - 1;
			if( hm>max )
				max=hm, maxi=i;
			else if(hm==max)
				maxi=-1;
		}
		return maxi;
	}

	LL toLLl( string s, LL k )
	{
		if( s.size() > k )
			s.resize(k);
		else if( s.size() < k )
			s.resize(k, 'a'), s[s.size()-1]-=1;
		return toLL(s,k);
	}

	LL toLLr( string s, LL k )
	{
		if( s.size() > k )
			s.resize(k), s[s.size()-1]+=1;
		else if( s.size() < k )
			s.resize(k, 'a');
		return toLL(s,k);
	}

	LL toLL( string s, LL k )
	{
		LL n = 0;
		for(int i=0; i<k; ++i)
			n = n*26 + s[i]-'a';
		return n;
	}

	LL ipow(LL a, LL k) {
		LL n = 1;
		while(k--)
			n *= a;
		return n;
	}

// BEGIN CUT HERE
	public:
	void run_test(int Case) { if ((Case == -1) || (Case == 0)) test_case_0(); if ((Case == -1) || (Case == 1)) test_case_1(); if ((Case == -1) || (Case == 2)) test_case_2(); if ((Case == -1) || (Case == 3)) test_case_3(); if ((Case == -1) || (Case == 4)) test_case_4(); if ((Case == -1) || (Case == 5)) test_case_5(); }
	private:
	template <typename T> string print_array(const vector<T> &V) { ostringstream os; os << "{ "; for (typename vector<T>::const_iterator iter = V.begin(); iter != V.end(); ++iter) os << '\"' << *iter << "\","; os << " }"; return os.str(); }
	void verify_case(int Case, const int &Expected, const int &Received) { cerr << "Test Case #" << Case << "..."; if (Expected == Received) cerr << "PASSED" << endl; else { cerr << "FAILED" << endl; cerr << "\tExpected: \"" << Expected << '\"' << endl; cerr << "\tReceived: \"" << Received << '\"' << endl; } }
	void test_case_0() { string Arr0[] = {"time","zoology"}; vector <string> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); int Arg1 = 1; int Arg2 = 0; verify_case(0, Arg2, likely(Arg0, Arg1)); }
	void test_case_1() { string Arr0[] = {"az","ma","xz"}; vector <string> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); int Arg1 = 1; int Arg2 = 1; verify_case(1, Arg2, likely(Arg0, Arg1)); }
	void test_case_2() { string Arr0[] = {"az","ma","xz"}; vector <string> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); int Arg1 = 2; int Arg2 = 2; verify_case(2, Arg2, likely(Arg0, Arg1)); }
	void test_case_3() { string Arr0[] = {"a","m","y"}; vector <string> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); int Arg1 = 1; int Arg2 = -1; verify_case(3, Arg2, likely(Arg0, Arg1)); }
	void test_case_4() { string Arr0[] = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y"}; vector <string> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); int Arg1 = 1; int Arg2 = 25; verify_case(4, Arg2, likely(Arg0, Arg1)); }
	void test_case_5() { string Arr0[] = {"aa", "ab", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"}; vector <string> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); int Arg1 = 1; int Arg2 = 0; verify_case(5, Arg2, likely(Arg0, Arg1)); }

// END CUT HERE
};
// BEGIN CUT HERE 
int main() { LikelyWord().run_test(-1); }
// END CUT HERE
