Inconsistent summation in C

When I run the following program:

#include <stdio.h>
#include <math.h>

int main()
{
    double sum, increase;
    long amount, j;

    printf("sum = ");
    scanf("%lf", &sum);
    printf("increase = ");
    scanf("%lf", &increase);
    printf("amount = ");
    scanf("%ld", &amount);

    for (j = 1; j <= amount; j++)
    {
        sum += increase;
    }

    printf("%lf\n", sum);

    return 0;
}

I get the following answer for these values:

MacBook:c benjamin$ ./test
sum = 234.4
increase = 0.000001
amount = 198038851
432.438851
MacBook:c benjamin$ ./test
sum = 234.4
increase = 0.000001
amount = 198038852
432.438851
MacBook:c benjamin$ ./test
sum = 234.4
increase = 0.000001
amount = 198038853
432.438852

where I increased the variable "quantity" by 1 in each case.

• In the first case, the summation gives what I expect.
• In the second case, this unexpectedly gives the same meaning.
• In the third case, he goes to the summation.

Why is this happening?

Although the code is not very useful, I just wrote this part. I really wanted to use it in a larger program.

Thank!