Gas Station

class Solution:
    # @param gas, a list of integers
    # @param cost, a list of integers
    # @return an integer
    def canCompleteCircuit(self, gas, cost):
        additional_gas=[]
        for i in range(len(gas)):
            additional_gas.append(gas[i]-cost[i])
        if sum(additional_gas)<0:
            return -1
        start=0
        for i in range(1,len(gas)):
            if (additional_gas[i]>=0 and additional_gas[i-1]<0):
                start=i
        return start