看看这串代码的错误

#include <stdio.h>

int main()

{

int num;

int sum;

int a = 31;

int b = 32;

int c = 41;

scanf("%d%d\n",num,sum);

if(num = a)

{

if(sum = b)

{

printf("%d\n",c);

}

}

return 0;

}

为什么不能实现输入31，32时输出41