數值計算不對

請問, 為何如下算出來的ThreeMonthRevenue數值為0

variable:_value1(0),_value2(0),_value3(0),ThreeMonthRevenue(0);

_value1 = getfield("月營收", "M")[0];

_value2 = getfield("月營收", "M")[1];

_value3 = getfield("月營收", "M")[2];

ThreeMonthRevenue = (_value1+_value2+_value3)/3;