c++ - sum elements of map by iterator -

i want sum first 4 elements of map using iterator in c++.

map<int, string> numbers; //making object of map numbers.insert(pair<int, string>(2,"two")); //inserting values map numbers.insert(pair<int, string>(3,"two")); //inserting values map numbers.insert(pair<int, string>(4,"two")); //inserting values map numbers.insert(pair<int, string>(7,"two")); //inserting values map numbers.insert(pair<int, string>(5,"two")); //inserting values map map<int, string>::iterator it; it=numbers.begin(); for( ;it!=numbers.begin()+4; it++){ //problem here , in body  } 

you can use std::accumulate <algorithm> header:

std::accumulate(it, std::next(it,4), 0,                  [](const auto& a, const auto& b){return + b.first;});