错误提示 error C2181: illegal else without matching if，而且是最后一个else处错误，求高手解答

#include int main() { double a, b, c, x1, x2, dise; printf("input a b c:"); scanf("%lf%lf%lf", &a, &b, &c); if (a == 0) printf("该方程不是一元二次方程"); else { dise = b * b - 4 * a * c; if (dise == 0) printf("有两个相等的实根: