The most valuable gift I have ever received was a photo album from my best friend Kevin.

      Kevin and I were classmates for all six years of primary school. We both had the same interests, so we always had a lot of things to talk about. It is wonderful to have such a good friend.

      However, last summer, Kevin had to move to another city because of his father's job. Then he gave me a photo album. “It has photos of the memorable events we have experienced together. I've also written some notes under each photo,” he said. I was deeply moved.

      I put the album beside my bed. I always look at it when I think of Kevin.

对于复数a,b,c,d，若集合S={a,b,c,d}具有性质“对任意$x,y\in S$，必有$xy\in S$”，则当$\{\begin{array}{c}a=1\\ b^2=1\\ c^2=b\end{array}$时，b+c+d等于(　　)