杭电acm2401

#include <stdio.h>int main(){int n, w, d, sum, ans, t;while(~scanf("%d%d%d%d", &n, &w, &d, &t)){     sum = n * (n-1) / 2 * w;     ans = (sum - t) / d;     if(sum==t)       printf("%d\n", n);        else     printf("%d\n", ans);}return 0;}