#include <vector>

#include <numeric>

int gasStations(std::vector<int>& gas, std::vector<int>& cost) {

// If the total gas is less than the total cost, completing the

// circuit is impossible.

if (std::accumulate(gas.begin(), gas.end(), 0) < std::accumulate(cost.begin(), cost.end(), 0)) {

return -1;

}

int start = 0;

int tank = 0;

for (int i = 0; i < gas.size(); i++) {

tank += gas[i] - cost[i];

// If our tank has negative gas, we cannot continue through the

// circuit from the current start point, nor from any station

// before or including the current station 'i'.

if (tank < 0) {

// Set the next station as the new start point and reset the

// tank.

start = i + 1;

tank = 0;

}

}

return start;

}