var offsets = require("fs").readFileSync("input.txt").toString().split("\n").filter((a)=>(a)).map((a)=>(parseInt(a)));

var pc = 0;
var i = 0;
while(offsets[pc] != null){
	i++;
	var oldpc = pc;
	pc += offsets[pc];
	if(offsets[oldpc] >= 3){
		offsets[oldpc]--;
	}
	else{
		offsets[oldpc]++
	}
}
console.log(i);