Postfix expression evaluation

[HelenI]

Hello there!! I have to solve a postfix expression evaluation.
e.g. : 7 4 -3 * 1 5 + / *
the answer is -14
my code is always printing 55.... i need help.

  #include<iostream>
#include<string>
#include<stack>
using namespace std;

int main(){
    int i,x,y;
    string a;
    stack <int> b;

    cin>>a;

    for(i=0;i<a.size();i++){
        if(a[i]!= '*' || '+' || '/' || '-'){
            b.push(a[i]);
        }
        else if(a[i]=='*'){
               x=b.top();
               b.pop();

               y=b.top();
               b.pop();

               a[i]=x*y;
               b.push(a[i]);

        }
        else if(a[i]=='/'){
               x=b.top();
               b.pop();

               y=b.top();
               b.pop();

               a[i]=x/y;
               b.push(a[i]);

        }
          else if(a[i]=='+'){
               x=b.top();
               b.pop();

               y=b.top();
               b.pop();

               a[i]=x+y;
               b.push(a[i]);

        }
        else if(a[i]=='-'){
               x=b.top();
               b.pop();

               y=b.top();
               b.pop();

               a[i]=x-y;
               b.push(a[i]);

        }

    }
    cout<<b.top();
}