// verifast_options{disable_overflow_check target:ILP32}
extern void abort(void);
//@ requires true;
//@ ensures true;


void reach_error()
//@ requires false;
//@ ensures true;
{}
extern void __VERIFIER_assert(int cond);
void __VERIFIER_assert(int cond) 
//@ requires (cond != 0);
//@ ensures (cond != 0);
{  if (!(cond)) {
    ERROR: {reach_error();abort();}
  }
  return;
}
extern int __VERIFIER_nondet_int();
//@ requires true;
//@ ensures true;
int main() 
//@ requires module(sum01_2__verifast_instrumented, true);
//@ ensures junk();
{
    //@ open_module(); 
  int i, n=__VERIFIER_nondet_int(), sn=0;
  if (!(n < 1000 && n >= -1000)) return 0;
for(i=1; i<=n; i++)
//@ invariant (((sn == 0) && (i == 1)) || ((sn <= (n * 2)) && ((2 + sn) == (i * 2))));
    {
    sn = sn + (2);
  }
  __VERIFIER_assert(sn==n*(2) || sn == 0);
 return 0; }