fractional knapsack problem

fKnapSack.cpp

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include<utility>
#include<iomanip>
using namespace std;

 bool myfunction( pair<int,int> a,pair<int,int> b)
  {
       return ((double)a.second/a.first) >((double)b.second/b.first) ;  
  }
  void mygold(vector< pair<int,int> > &samples,int N,int W)
  {
      sort(samples.begin(),samples.end(),myfunction);
     
      double myMoney=0;int i;
      for(i=0 ; (i<N) && ( W );i++)
      {
          if(W>=samples[i].first)
          {
              myMoney+=(double)samples[i].second;
              W-=samples[i].first;
          }
          else
          {
              myMoney+=(W)*((double)samples[i].second/(double)samples[i].first);
             W=0;
          }
      }
      if((i==N)&&(W!=0))
        cout<<-1;
      else {std::cout << std::fixed;
      cout<<std::setprecision(12)<<myMoney;
           }
  }
int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    int N,W,w,v;
    cin>>N>>W;
     pair<int,int>s;
    vector< pair<int,int> > samples;
    int sum=0;
    for(int i=0;i<N;i++)
    {
        cin>>w>>v;
        sum+=w;
        s=make_pair(w,v);
        samples.push_back(s);
    }
    
    mygold(samples,N,W);    
    return 0;
}