void abort() { };

                                                                       
                                                  
                                                                             
                                                                   
                                                  
                                                                             
                                                                              
                                                                             

                                                                                                                                                               
                        
void assume_abort_if_not(int cond) {
  if(!cond) {abort();}
}
/*@ 
    requires ((1 <= cond)) && (cond != 0);
    ensures ((1 <= cond)) && (1);
@*/
void __VERIFIER_assert(int cond) {
  if (!(cond)) {
  ERROR: {/*@ assert(0); */;abort();}
  }
  return;
}
int __VERIFIER_nondet_int();
int main() {
    int n, sum, i;
    n = __VERIFIER_nondet_int();
    if (!(1 <= n && n <= 1000)) return 0;
    sum = 0;
/*@
loop invariant ((((i + ((long long) sum * 2)) == ((long long) i * i)) && (((n + ((long long) n * n)) == ((long long) sum * 2)) || (i < ((long long) n + 1)))));
@*/
    for(i = 1; i <= n; i++) {
        sum = sum + i;
    }
    __VERIFIER_assert(2*sum == n*(n+1));
    return 0;
}