很久没编了～哎～
哪位大侠帮忙看看～那个边值问题没考虑到而导致这里不通过的。

自己的源程序：
#include<stdio.h>
main()
{
	long x, y, m, n, l, dist, k, t;
	int flag;

	do
	{
		scanf("%ld %ld %ld %ld %ld", &x, &y, &m, &n, &l);
	}
	while( x == y );                  // 这里是以防输入x = y的语句，要求重新输入。
    
	if( m > n )
		flag = 1;
	else if( m == n )
		flag = 0;
	else
		flag = -1;                    // 设置标志。

	switch( flag )
	{
	case 0: 
		printf( "Impossible\n" );
	
	case 1:
		if( x > y )
			dist = l - ( x - y );
		else
			dist = y - x; 

		for( k = 0; k < ( m - n ); k++ )
		{
			if( ( dist + k * l ) % ( m - n ) == 0 )
			{
				t = ( dist + k * l ) / ( m - n );
				printf( "%ld\n", t );
				break;
			}
			if( k == ( m - n - 1 ) )
				printf( "Impossible\n" );
		}

	case -1:
		if( x > y )
			dist = x - y;
		else
			dist = l - ( y - x );

		for( k = 0; k < ( n - m ); k++ )
		{
			if( ( dist + k * l ) % ( n - m ) == 0 )
			{
				t = ( dist + k * l ) / ( n - m );
				printf( "%ld\n", t );
				break;
			}
			if( k == ( m - n - 1 ) )
				printf( "Impossible\n" );
		}
	}
	return 0;
}