Let us consider 2 odd numbers, let’s say 2n+1 and 2m+5. As you see for any integral value of n and m, we have the above numbers odd. When we add them,
2n +1 +2m+5 =2(n+m) +6= 2(n+m+3)= EVEN
No matter what is the sum of m and n, Since it got multiplied by 2 it's even and 6 is another even . So the total sum is even.