func evalRPN(tokens []string) int {
    stack:=[]int{}

    operators:= map[string]func(int,int) int {
        "+": func (a,b int) int { return a + b},
        "-": func(a, b int) int { return a - b },
        "*": func(a, b int) int { return a * b },
        "/": func(a, b int) int { return a / b },
    }

    for _,token:= range tokens {
        if calc,f:= operators[token]; f {
            a,b:=stack[len(stack)-2],stack[len(stack)-1]
            val:= calc(a,b)
            stack = append(stack[:len(stack)-2], val)
        }else {
            num,_:= strconv.Atoi(token)
            stack=append(stack,num)
        }
    }
    return stack[0]
}