#include <iostream>

using namespace std;

int main(){
    int nt, mt, om, nm, x = om;
    cin >> nt >> mt >> om >> nm;

    for(int i = 0; i < mt; i++){
        if(x != 0){
            x--;
        }  
        if(i == nt-1){
            x += nm;
        }     
    }
    cout << x << endl;
}