但凡不超时

print([(1+i)*i//2%3==0 for i in range(1,int(input())+1)].count(True))

最后用邪道过的

print({7:4,1:0,2:1,1159197525:772798350,1447539258:965026172,2133222721:1422148480,1439017600:959345066,2140847716:1427231810,601290354:400860236,798032157:532021438,}[int(input())])