/*
PROOF OF CORRECTNESS:
 NEGATE(N): compute negative of a non-negative integer, e.g. NEGATE(10) = -10
*/
#include <stdio.h>


int negate(int x) {
int i,f;

  // PRE: x>=0
  i = 0;
  


  f = x;
 
 

  // INVARIANT:


  while ( i < 2*x ) {
   

 
    f = f - 1;
   

 

    i = i + 1; 



  }  

  // INVARIANT:                             and EXIT: 
  






  // POST: f=x-2*x
  return f;
}

int main() {
  printf("negate(+10)=%d\n",negate(10));
}