what it means by - "void value not ignored as it ought to be "?

#include<bits/stdc++.h> using namespace std;

int prefixEvalution(string chk){ stackst;

for(int i=chk.length()-1;i>=0;i--){

    if(chk[i]>='0' && chk[i]<='9'){

        st.push(chk[i]-'0');
    }
    else{
        **int a=st.pop();
        int b=st.pop();**

        switch(chk[i]){

        case '+':
            st.push(a+b);
            break;

        case '-':
            st.push(a-b);
            break;

        case '*':
            st.push(a*b);
            break;

        case '/':
            st.push(a/b);
            break;

        case '^':
            st.push(pow(a,b));
            break;
        default:
            break;
        }

    }
}
return st.top();

}

Here compiler shows an error for "int a= st.pop();" and says "void value not ignored as it ought to be ". I don't know why?