hpgcc2 loop problem

[BINUBALL]

I use HPGCC2 and this tutorial to make sum program for 50g.
http://sense.net/~egan/hpgcc/
example, This sum program made in UserRPL looks like this

Code:

<< -> n
<< 0. 1. n FOR X X + NEXT
>>
>>

It's simple program, which gets stack 1 number and calculate 1 to that number sum.

so, I want to C's speed. I coded this.

Code:

#include <hpgcc49.h>

int main(void)
{
    // sum 1 to n and counter i
    int n, i;
    int sum = 0;
    // fast mode
    sys_slowOff();
    // get stack 1 real number
    n = sat_pop_real();
    // calculate sum
    for (i = 1; i <= n; i++) {
        sum += i;
    }
    // output
    sat_push_real(sum);
    sys_slowOn();
    return(0);
}

and based on top tutorial, I made UserRPL wrapper.

Code:

<< "Input 1 real" -> n
<< DEPTH IF 0 == THEN u DOERR END 3: "EXTEND/SM" EVAL
>>
>>

where "EXTEND/SM" is C program's location.

But I execute this program to calculate 1 to 1000 sum, It gives 499500 instead of 500500. and number goes up over 1000, It gives wrong answer.
ex)
input -> answer
999 -> 499,500 (correct)
1,000 -> 499,500 (incorrect which is 500,500)
1,001 -> 500,500 (incorrect which is 501,501)
2,000 -> 1,999,000 (incorrect which is 2,001,000)
10,000,000 -> -2,001,260,032 (I think it is overflow)

I'm a very noob for C, so my code may be wrong. What's wrong with my program?