I can't understand what's wrong in my code, it write 51 instead of 50

#include <iostream>
using namespace std;

int main()
{
    unsigned short a, b, c;

    cin >> a >> b;

    c = (b - a) + 1;

    if(c % 2 == 0)
        cout << c / 2;
    else
        cout << (c / 2) + 1;  /// 1! 2 3! 4 5! 6 7! 8 9! 10 11! 12 13! 14 15!

    return 0;
}