Compilation Error: Pairs and xutility

Hello, all! I am working on this program to find a solution to the traveling salesman problem. I am very frustrated because it doesn't seem to compile. I am getting this error:

error C2039: 'iterator_category' : is not a member of 'std::pair<_Ty1,_Ty2>'

It is in my "totalDis(...)" method. When I try to call "distance(...)" with two pairs from my vector, it tells me there are errors in xutility. There are a ton of errors, but the one above encapsulates them all. I am hoping the error is something obvious that I am missing.

Thank you for the help!

#include <fstream>
using std::ifstream;

#include <iostream>

#include <cmath>

#include <string>
using std::string;

#include <utility>
using std::make_pair;
using std::pair;

#include <vector>
using std::vector;

typedef pair< double, double > point;
typedef vector< point > graph;
typedef unsigned long ulong;

//----------------------------------------------------------

// pre: ifstream is a file containing the number of
//        coordinates to read in and the number of coordinates
// pst: returns vector of coordinate pairs
graph readFile( ifstream & );
// pre: graph is a vector of coordinate pairs
// pst: returns a vector of coordinate pairs, representing
//        the TSP path that visits each node exactly once
graph slowTSP( const graph & );
// pre: graph is a vector of coordinate pairs
// pst: returns a vector of coordinate pairs, representing
//        the TSP path that visits each node exactly once
graph fastTSP( const graph & );
// pst: returns the linear distance between two
//        coordinate points
const double distance( const point, const point );
// pst: returns the total linear distance of the cycle
//        that visits each point exactly once
const double totalDis( const graph & );

//----------------------------------------------------------

const string FILENAME( "input.txt" );

int main( int num, char **args )
{
    ifstream fin( FILENAME.c_str() );

    graph vec = readFile( fin );

    // print coordinates
    for (int i=0; i<vec.size(); ++i)
        std::cout << "(" << vec[i].first << ", "
                  << vec[i].second << ")\n";

    return 0;
}

//----------------------------------------------------------

graph readFile( ifstream & fin )
{
    graph v;
    ulong size;
    fin >> size;    // get number of coordinates

    double x;
    double y;
    for (int i=0; i<size; ++i) {
        // construct list of coordinate pairs
        fin >> x >> y;
        v.push_back( make_pair(x, y) );
    }

    return v;
}

//----------------------------------------------------------

graph slowTSP( const graph & in )
{
    graph min( in );

    return min;
}

//----------------------------------------------------------

graph fastTSP( const graph & in )
{
    graph v( in );

    return v;
}

//----------------------------------------------------------

const double distance( const point a, const point b )
{
    // a^2 + b^2 = c^2
    return (sqrt( pow((a.first - b.first), 2) +
                  pow((a.second - b.second), 2)));
}

//----------------------------------------------------------

const double totalDis( const graph & in )
{
    double dis = 0;
    // distance from start to end point
    for ( int i=0; i<in.size(); ++i) {
        double temp = distance(in.at(i-1), in.at(i));
        dis += temp;
    }
    // distance from end point back to start
    dis += distance( in.at(in.size()-1), in.at(0) );

    return dis;
}

//----------------------------------------------------------