S=1+9i^4+17i^8+...+97i^48
-3i-11i^5-19i^9-...-99i^49
+5i^2+13i^6+21i^10+...+93i^46
-7i^3-15i^7-23i^11-...-95i^47
=(1+9+17+..+97)-i(3+11+19+..+99)+i^2(5+13+21+..+93)-i^3(7+15+23+..+95)
=(1-5+9-13+17-21+...-93+97)-i(3-7+11-15+19-23+...-95+99)
=(1+4+4..+4)-i(3+4+4...+4)
=49-51i