Task 14 - C - Getting wrong solution and unable to locate why

Here is my code:

#include <iostream>
#include <fstream>

using namespace std;

int main() {
    unsigned long long base = 0;

    ifstream input("input.txt");
    ofstream output("output.txt");

    input >> base;

    while (!input.eof()) {
        char action = '0';
        long int number = 0;

        input >> action;
        input >> number;

        switch(action) {
        case '+':
            base += number;
            break;
        case '*':
            base *= number;
            break;
        case '%':
            base %= number;
            break;
        };
    }

    output << base;

    input.close();
    output.close();
}

I'm absolutely stuck on this task and can't seem to locate the cause of the wrong answer. It works on the example test data, but not on the actual test data that I'm given.

Now that you say that, it seems somewhat obvious that the modulo can be applied to all steps to prevent the number from getting too big. Thanks a bunch!