#include <stdio.h>
#include <string.h>
int map[28][28],num,n;
char output[28];
int Floyd()
{
	int i,k,j;
	for(i=1;i<=n;i++)
		for(k=1;k<=n;k++)
			for(j=1;j<=n;j++)
				if( map[k][i] && map[i][j] )
					map[k][j]=1;
	for(i=1;i<=n;i++)
		if(map[i][i]==1)
			return 1;
	return 0;
}
int Topsort()
{
	int cou,ind,k,i,degree[28],used[28];
	num =0;
	memset(used,0,sizeof(used));
	memset( degree,0,sizeof(degree) );
	for(i=1;i<=n;i++)
		for(k=1;k<=n;k++)
			if(map[i][k])
				degree[k]++;
	for(i=1;i<=n;i++)
	{
		cou=0;
		for(k=1;k<=n;k++)
			if(degree[k]==0 && !used[k])
			{
				output[num++]=k+‘A‘-1;
				used[k]=1;
				cou++;
				ind=k;
			}
		if(cou>1)  return 0;
		for(k=1;k<=n;k++)
			if(map[ind][k]) degree[k]--;
	}
	output[num]=‘\0‘;
	return 2;
}
int main()
{
	int m,flag,i,pos;
	char str[4];
	while( scanf("%d%d",&n,&m)!=EOF && ( n || m ) )
	{
		flag=0;
		memset(map,0,sizeof(map));
		for(i=1;i<=m;i++)     //每输入一条边就进行拓扑排序和判断有没有环
		{
			scanf("%s",str);
			map[str[0]-‘A‘+1][str[2]-‘A‘+1] = 1;
			if(flag) continue;
			flag=Floyd();
			if(flag==1)
				pos=i;
			else if(flag==0)
				flag=Topsort();
			else
				continue;
			if(flag==2)
				pos=i;
		}
		if(flag==2)
			printf("Sorted sequence determined after %d relations: %s.\n",pos,output);
		else if(flag==1)
			printf("Inconsistency found after %d relations.\n",pos);
        else
		printf("Sorted sequence cannot be determined.\n");
	}
	return 0;
}

Sorting It All Out(拓扑排序，floyd）