#include <iostream>
using namespace std;
int taxi(int a,int b,int c,int d){
if ((a+b)>c) {return c;}
a+=b;
if ((c-d)<a) return a;
c-=d;
taxi(a,b,c,d);
}
int main()
{
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    cout<<taxi(a,b,c,d);
    return 0;
}

Wrong answer test #1

"<=" and ">=" in those ifs.
there is one more case where a >= c where you need to return a.