c语言求教：illegal else without matching if 是什么意思呀，应该怎么改呢

if后面的程序包没有用{}包围起来 #include #include int main() { double a,b,c; scanf("%lf\n",a); if (a>1000){ printf("please input again"); scanf("%lf\n",a); b=sqrt(c); printf("%7.1f\n",b); } else{ b=sqrt(a); printf