Reverse Polish notation evaluation

#include <iostream>
#include <string>
#include <stdlib.h>
#include <stdio.h>
using namespace std;

int top = 0, S[1000];

int pop()
{
    top--;
    return S[top + 1];
}

void push(int x)
{
    S[++top] = x;
}

int main()
{
    stack<int>st;

    char s[100];
    while(scanf("%s", s) != EOF)
    {

        if(s[0] == '+')
        {
            int a = pop();
            int b = pop();
            push(a + b);
        }
        else if(s[0] == '*')
        {
            int a = pop();
            int b = pop();
            push(a * b);
        }
        else if(s[0] == '-')
        {
            int a = pop();
            int b = pop();
            push(b - a);
        }
        else
        {
            push(atoi(s));
        }
    }

    printf("%d\n", S[1]);
    return 0;
}