How to discover where code ends.

function  days  = day_diff( M1,D1,M2, D2 )
%DAY_DIFF Summary of this function goes here
%   Detailed explanation goes here
if M1 ~= 1 && M1 ~= 2 && M1 ~=3 && M1 ~= 4 && M1 ~= 5 && M1~=6 && M1~=7 && M1~=8 && M1~=9 && M1~=10 && M1~=11 && M1 ~= 12
days = -1;
return;
end
if M2 ~= 1 && M2 ~= 2 && M2 ~=3 && M2 ~= 4 && M2 ~= 5 && M2~=6 && M2~=7 && M2~=8 && M2~=9 && M2~=10 && M2~=11 && M2 ~= 12
days = -1;
return;
end
if D1 > 31 || D1 < 1
days = -1;
return;
end
if D2>31 || D2<1
    days = -1
    return;
end

if M1 ==1 || M1 ==2 || M1 ==3 || M1 ==4
 if D1 <1 || D1 >30
 days = -1;
 return;
end
end
if M2 ==1 || M2 ==2 ||M2 ==3 || M2 ==4
 if D2 <1 || D2 >30
 days = -1;
 return;
end
end
total =0;
if M2<M1
    for count=M2+1:M1-1
        if M1==4 ||M1==6||M1==9||M1==11
            add = 30;
        else
            add = 31;
        end
        total = total + add;
    end
end
if M2<M1
    if M2 == 4 || M2 == 9 || M2 == 11 || M2==6
        diff = 30-D2;
    else
        diff = 31-D2;
    end
end

if M1<M2
    if M1 == 4 || M1 == 9 || M1 == 11 || M1==6
       diff = 30-D1;
    else
        diff = 31-D1;
    end
end
total =0;
if M2>M1
    for count=M1+1:M2-1
    if count == 4 || count == 9 || count == 11 || count==6
        days2 = 30;
    else
        days2 = 31;
    end
    total = total + days2;
    end
end        
if M1>M2
    for count=M2+1:M1-1
    if count == 4 || count == 9 || count== 11 ||count==6
        days2 = 30;
    else
        days2 = 31;
    end
    total = total+days2;
end    

if M1 > M2
    days = total + diff + D1;
else
    days = total + diff + D2;
end

end

How do I discover where Matlab believes this code ends? Its telling me I assign no value to "days" in the code, but I do at the end.