can you puzzle it out ?

Puzzle it out !!

The black and white beach:
Here is a mystery from long long ago ,
In ancient times ,there were two nations called Atlantis and Mediterrains and they were always at war .Each nation had aroyal family .king Harold of mediterrains was a good king but king Gerald of atlantis was an evil king. Gerald wanted to be king of both nations .One day ,evil king Gerald captured good king Harold and held him prisoner .harlod's daughter princess Rose pleaded with king Gerald for the of her father .Gerald sent a message to the princess .the notes said :'if you want to save the life of your father , meet me on the black and white beach at midnight .'
The black and white beach was the border between the two countries . it was a long beach covered in pebbles. all the pebbles were black or white. Princess Rose went to the beach with two body guards .Gerald was there alone .He was carring a bag ."Do you want to save the life of your father?", asked king Gerald. "Of course", said princess Rose."In that case , this is what I suggest ", said king Gerald , smiling in an evil way ."I will put one black pebble and one white pebble in my bag . you must take one pebble out of the bag. if you choose the white pebble, your father will go free. But if you choose the black one , you must marry me , and I will be ruler of both our kingdoms". Princess rose reluctantly agreed to the bargain .in the darkness king Gerald picked two pebbles from the beach and put them in his bag . but there was aproblem . one of rose's body guards whispered in her ear. "princess,the king put 2 black pebbles in the bag!" princess rose realized that she had been tricked . But she knew that if she accused king Gerald of cheating ,he would be angry and she wouldn't be able to save her father.

How did princess rose manage to save her father and avoid marrying evil king Gerald????