Preferred language using C++//

Basically the expression below has been converted into a C++ postfix expression. Now i need to reckon the result.

I used deque somehow, instead of stack, because i tried using stack and it didn't seem to work. but there's bug.

7
8 9 + 1 7 - *

-102

Here's my code:

#include <fstreaM>
#include <string>
#include <deque>

using namespace std;

int main(){
ifstream inf("input.txt");
ofstream outf("output.txt");

string line; inf >> line;

deque <int> stack;

for(int i = 0; i < line.length(); i+=2){
// char ch = line.at(i);
if(line.at(i) == '*'){
stack.push_back(stack.pop_back() * stack.pop_back());
}else if(line.at(i) == '+'){
stack.push_back(stack.pop_back() + stack.pop_back());
}else if(line.at(i) == '-'){
stack.push_back(-stack.pop_back() + stack.pop_back());
}else stack.push_back(line.at(i)- '0');

}

outf << stack.pop_back() << endl;

return 0;
}

With the code above can i alter with a small tweak and have it change into a Prefix Notation.

I still prefer using the std::deque STL Library from C++.