simulink sum/add's output is wrong

Hi,

I'm simulating a fractional frequency synthesizer. And there is a mash 1-1-1 delta-sigma modulator(Graph 1) in it. But I find that the output of "sum(or add)" module is wrong. For example(Graph 2), one input is 0/1 wave and the other input is 0/-1 wave, but the output is 0/-2 wave, not 1/-1 wave. I don't know why. Did I set something wrong? Any advice will be appreciated.

Thanks,

Yi