C++ 容器 Vector

//读入一组整数到vector对象，计算并输出每对相邻组合的和。如果读入元素个数为奇数，则提示用户最后一个元素没有求和，并输出其值。

#include <iostream>

#include <string>
#include <vector>
using namespace std;

int main(void)
{
vector<int> int_vec;
int i;

while (cin>>i)
{
int_vec.push_back(i);
}

if(int_vec.size() % 2 == 0)
{
for (vector<int>::size_type index=0; index != int_vec.size(); index=index+2)
{
cout<<"The sum is : "<<int_vec[index] + int_vec[index+1]<<endl;
}
}

else if (int_vec.size() % 2 == 1)
{
for (vector<int>::size_type index=0; index <= int_vec.size()-3; index=index+2)
{
cout<<"The sum is : "<<int_vec[index] + int_vec[index+1]<<endl;
}
vector<int>::size_type index=int_vec.size();
cout<<"The last element is not sumed and  its value is  : "<<int_vec[index-1]<<endl;
}

system("pause");
return 0;
}