运算结果不解

#include <stdio.h>

int main()

{

    int a,b,c,d,result;

    double result1,result2;

    a = 1;

    b = 2;

    c = 3;

    d = 4;

    result = a + b - c * d;    //在这里体验哦~

    result1 = (b - c) / (a + d);

    result2 = d * (a - d) / c;

    printf("%d\n", result);

    printf("%f\n",result1);

    printf("%f\n",result2);

    return 0;

}

为什么我运行计算结果 result1 是0.000000 ???