int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {

int current = 0;

int start = gas.size(); //start from the end to beginning

int total = 0;

do {

if (total + gas[current] - cost[current] >= 0) {

total += (gas[current] - cost[current]);

current++; // can go from current to current+1

}else{

start--; //not enough gas, try to start the one before origin start point.

total += (gas[start] - cost[start]);

}

} while(current != start);

return total>=0 ? start % gas.size() : -1;

}