// tested by Hightail - https://github.com/dj3500/hightail
import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;

immutable inf = 10^^9 + 7;

int c, v0, v1, a, l;

void main() {
    scan(c, v0, v1, a, l);

    int r;
    int cnt;

    while (true) {
        cnt++;
        r += min(v0 + a * (cnt - 1), v1);

        if (r >= c) {
            writeln(cnt);
            return;
        }

        r -= l;
    }
}


void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}


void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}