Re: Very strange problem with a operator "-"

I was trying to solve: acm.uva.es/p/v101/10191.html

I did this code:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>
#include <cctype>
using namespace std;

bool digit(char c)
{
  return isdigit(c);
}

struct appointment
{
int h1; // beginning hour of app.
int m1; // beggining minute of app.
int h2; // ending hour of app.
int m2; // ending minute of app.
};

// compare appointments by their beggining hour
bool lt(appointment a1,appointment a2)
{
  return (a1.h1)<(a2.h1);
}

int main()
{
string n; // number of appointments in a day
while(getline(cin,n))
{
   vector<appointment>agenda;
   int app = atoi(n.c_str());
   for(int times=1; times<=app; ++times)
   {
      string line;
      getline(cin,line);
      // appointments are given in format h1:m1 h2:m2 blah, blah

      appointment aux;
      aux.h1 = atoi(line.c_str());
      line.erase(line.begin(),line.begin()+line.find(':')+1);
      aux.m1 = atoi(line.c_str());
      line.erase(line.begin(),line.begin()+line.find(' ')+1);
      aux.h2 = atoi(line.c_str());
      line.erase(line.begin(),line.begin()+line.find(':')+1);
      aux.m2 = atoi(line.c_str());
      agenda.push_back(aux);
   }
   sort(agenda.begin(),agenda.end(),lt);
   // all appointments begin at 10:00 and end at 18:00
   // time elapsed between 10:00 and first appointment
   cout << agenda.front().h1*60+agenda.front().m1 - 600 << endl;
   for(int i=0; i<agenda.size()-1; ++i)
   {
   // time elapsed between i-th and i+1-th appointments
   cout <<
agenda[i+1].h1*60+agenda[i+1].m1)-(agenda[i].h2*60+agenda[i].m2)
        << endl;
   }
   // time elapsed between last appointment and 18:00
   cout << 1080-agenda.back().h2*60+agenda.back().m2<< endl << endl;

}
return 0;
}

My problem is with line:
// time elapsed between last appointment and 18:00
cout << 1080-agenda.back().h2*60+agenda.back().m2<< endl << endl;

When the last input for the day is
XX:XX 17:45
It prints 105! As a matter of fact unless the last time is 18:00 it
always prints a wrong number!

What's more strange is if I do:

// time elapsed between last appointment and 18:00
cout << agenda.back().h2*60+agenda.back().m2-1080 << endl << endl;

It prints the correct negative number!

What's happening, here?!

Thanks.