double b[maxn][maxn];
#define zero(x) ((x>0? x:-x)<1e-15)
double det(double a[][maxn],int n) {
int i, j, k, sign = 0;
double ret = 1, t;
for (i = 0; i < n; i++)
for (j = 0; j < n; j++)
b[i][j] = a[i][j];
for (i = 0; i < n; i++) {
if (zero(b[i][i])) {
for (j = i + 1; j < n; j++)
if (!zero(b[j][i]))
break;
if (j == n)
return 0;
for (k = i; k < n; k++)
swap(b[i][k], b[j][k]);
sign++;
}
ret *= b[i][i];
for (k = i + 1; k < n; k++)
b[i][k] /= b[i][i];
for (j = i + 1; j < n; j++)
for (k = i + 1; k < n; k++)
b[j][k] -= b[j][i] * b[i][k];
}
if (sign & 1)
ret = -ret;
return ret;
}