#include "testlib.h"
using namespace std;

int main(int argc, char **argv) {
    registerValidation(argc, argv);
    
    while (!inf.seekEof()) {
        int n = inf.readInt(2, 8000, "n"); inf.readSpace();
        int w = inf.readInt(1, 30, "w"); inf.readSpace();
        int d = inf.readInt(1, w, "d"); inf.readSpace();
        int tot = inf.readInt(1, INT_MAX, "tot"); inf.readEoln();
        int x = w * (n * (n - 1) / 2) - tot;
        int ans = x / d;
        ensuref(0 <= x && x % d == 0, "impossible case");
        ensuref(0 <= ans && ans < n, "the light coins can't be in the n-th basket unless 'tot == w * (n * (n - 1) / 2) (i.e. ans == 0)'");
    }
    inf.readEof();
}