题解

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

int main(){
    int a, b, m, n, s[400] = {};
    cin >> a >> b >> m >> n;
    
    if(a > n)
        cout << a;
    else if(b > n)
        cout << a + b;

    s[0] = a;
    s[1] = b;
    
    int cnt = a + b;
    for(int i=2; i<n; i++){
        s[i] = s[i-1] + s[i-2];
        cnt += s[i];
        if(s[i] >= m) 
            break;
    }
    cout << cnt;
    return 0;
}