uva 10375 - Choose and divide(组合数）

#include <stdio.h>
#define min(a,b) (a)<(b)?(a):(b)

int main () {
int p, q, r, s;
while (scanf("%d%d%d%d", &p, &q, &r, &s) == 4) {
q = min(q, p - q), s = min(s, r - s);
double ans = 1.0;
for (int i = 1; i <= q || i <= s; i++) {
if (i <= q) ans = ans * (p - q + i) / i;
if (i <= s) ans = ans / (r - s + i) * i;
}
printf("%.05lf\n", ans);
}
return 0;
}