Scope of variable in if section

Why am getting NameError for "If" condition?

NameError

for i in range(int(n)):
        for j in range(int(k)):
            res = a1[i] + a2[j]
            if(res<=num and ((res-num)<(prev-num))):
                r1,r2 = a1[i],a2[j]
                prev = res

    print(r1,r2)

Running code snippet:

for i in range(int(n)):
        for j in range(int(k)):
            res = a1[i] + a2[j]

            if(res<=num):
                r1,r2 = a1[i],a2[j]
                prev = res

    print(r1,r2)

If I add "AND" in "IF" condition it gives NameError for variables defined under if statement. Could someone help on this ?

Thank you.