I think adat is a vector. Vector returns an unsigned integral type value, you can check this: vector's size()
You can ignore the warning or just use an unsigned int variable to iterate over vector. Or if you strongly need i to be a simple int variable you can roughly cast size to int
while(i < (int)adat.size()) {
but it causes problems if size is over MAX_INT value.
Furthermore, if you are using C++ 11 you can ignore the iterator variable in this way:
for(auto elem : adat) {
if (elem.korido < "01:30:000") {
s++;
}