1 + 2 + 3 + 4 + ... + 98 + 99 + 100 = (1 + 100) + (2 + 99) + (3+98) + ... + (48 + 53) + (49+52)+(50 + 51) = 101 * 50 = 5050
1+100=101; 101*50=5050
