Whats wrong with my code. I get correct answer for Test 6 on my compiler, but when I submit the solution, I get wrong answer. Please help!!

I tried with below values and got correct answer
4 3
3 4 5

Correct answer: 1

#include <iostream>
using namespace std;
int main()
{
    int a,k,n,d;
    int b=0;
    cin>>k>>n>>d;
    for (int i=0;i<n-1;i++)
    {
        cin>>a;
        a=a-k;
        b=b+a;
    }
    if (d-k>=0)
    {
        if (b+d-k>=0)
        cout<<b+d-k;
        else cout<<0;
    }


    else if (b>0) cout<<b;
    else cout<<0;
    char f;
    cin>>f;

}

I have some problem, try 5 4 3 , you must have 0.