import sys
def min_energy(n,a):
    prev2=a[0]
    prev1=prev2+a[1]
    for i in range(2,n):
        current=min(prev2,prev1)+a[i]
        prev2=prev1
        prev1=current
    return prev1
def main():
    n=int(sys.stdin.readline())
    a=list(map(int,sys.stdin.readline().split()))
    res=min_energy(n,a)
    print(res)
if __name__=="__main__":
    main()