1*1-2*2+3*3-4*4+5*5-6*6+…+49*49-50*50
=(1^2-2^2)+(3^2-4^2)+(5^2-6^2)+...+(49^2-50^2)
=-1*3-1*7-1*11-...-1*99
=-(3+7+11+...+99)
=-(3+99)*25/2
=-1275