100+99-98-97+96+95-94-93+......+8+7-6-5+4+3-2-1=(100-98)+(99-97)+(96-94)+(95-93)+......+(8-6)+(7-5)+(4-2)+(3-1)=2*50=100
100+99-98-97+96+95-94-93+......+8+7-6-5+4+3-2-1= (100 + 99 - 98 - 97) + (96 + 95 - 94 - 93) + ...... + (8+ 7 - 6 - 5) + (4 + 3 - 2 -1)= 4 + 4 + ...... + 4 + 4= 4 * 25= 100
100